I tried several flashes on my Canon EOS Rebel SL3, but it didn't work. I'm wondering which pin on hot shoe triggers the flash?
The T7/2000D, T100/4000D, and SL3/250D are all missing the big central sync connector on the hotshoe, which is the ISO standard signal for sync. This basically broke compatibility with any single-pin manual speedlight (e.g., YN-560 IV, or Godox TT600) which can only communicate that one signal.
And while firmware updates fixed compatibility issues with Godox's 2.4 GHz eTTL-capable speedlights for the T7 and T100, apparently those same updates do not work for the SL3. Only the Godox transmitters have received SL3-specific compatibility firmware updates.
If you need an affordable on-camera flash unit for the SL3, a Canon EL-100 is probably your best option.
But if you really need to use a 3rd-party single-pin flash, a Godox X1T-C or X2T-C transmitter can be used kind of like a hotshoe adapter to give you back the sync connection if you sandwich it between the hotshoe and the flash.
It is a combination of pins 2 and 3; when data (pin 3) is at logic high, and clock is held at logic low for 4ms (pin 2), then the flash fires.
Here's a DIY to add a microcontroller to an ISO standard speedlight in order to monitor pins 2-3 and trigger the flash appropriately. https://www.instructables.com/Generic-Speedlite-Hack-for-Canon-EOS-1500D/
Using my 80D and a 430EX II, I went through a process of elimination by covering the various hotshoe contacts with black electrical tape.
I was able to determine that contacts(pins) 2,3, and 4 are ALL needed in order for an ETTL flash to be triggered by a camera without the normal contact in the center.
Contact(pin) 5 is the only one that was not needed for the flash to fire. I am not sure what “AF Assist” means, but I believe it may communicate subject focus distance to the flash in order to fine tune the ETTL II flash exposure. Not all Canon lenses have this capability and it is not needed for ETTL to function.
Other than the Center contact, no other one contact by itself would trigger the flash to fire.
None of the contacts on the SL3 hot shoe trigger the flash in the way you seem to expect. That is, the large central contact that does what you describe is missing from the SL3 hot shoe.
None of the contacts included on the SL3 hot shoe complete a simple circuit that causes the flash to fire. The contacts on the SL3 hot shoe only work by communicating data to the TTL micro-processor in a compatible flash or flash trigger. The attached flash or flash trigger then uses the data sent to it via pins 2, 3, and 4 and applies its own logic circuits to decide when and at what power to "fire" the flash.