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Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7 II. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon. [It looks like the YN-622 triggers are incompatible with the Sony hotshoe, and the YN RT triggering is so new, it's an unknown quantity on Canon, let alone other brands].

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.

Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7 II. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon. [It looks like the YN-622 triggers are incompatible with the Sony hotshoe, and the YN RT triggering is so new, it's an unknown quantity on Canon, let alone other brands].

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.

Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7 II. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon.

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.

Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7 II. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon. [It looks like the YN-622 triggers are incompatible with the Sony hotshoe, and the YN RT triggering is so new, it's an unknown quantity on Canon, let alone other brands].

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.

Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon.

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.

Yes. There are Yongnuo flashes that will work with both systems. But only because they're manual-only flashes. You could find quite a few similar flashes in other brands as well (e.g., Lumopro LP180). Manual only flashes only require an ISO-compatible hotshoe, and both the 5DMkIII and A7 have those. You can purchase a Canon TTL-capable Yongnuo flash, and have those features work on the 5DMkIII, but it will be a manual-only flash on the A7 II. And vice-versa, if you pick up a Sony HVL speedlight model, it will only be useful as a manual-only flash on the 5DMkIII.

The YN-560III/IV flashes with a YN-560-TX controller. This combination gives you proper syncing of remote flashes over radio, with remote manual power and zoom control. Canon TTL models would include the YN-565EX, YN-568EX, YN-500EX, and YN-600EX-RT, among others. Yongnuo models proliferate like rabbits, but they only make TTL-capable models for Canon and Nikon.

There is no combination that will give you TTL on both systems. TTL by its very nature is proprietary and brand-specific, since it requires camera/flash electronic communication. If you want TTL on both systems, you're liable to have to purchase two separate speedlights.