If you want to use the built-in Canon RT radio system, then you need a unit that speaks that radio protocol. The only gear, aside from Canon's that does so, are new units from Yongnuo. Shanny is reputed to be working on RT compatible gear as well, but it hasn't yet been released. You could integrate flashes into an RT setup by using dumb optical triggers and the RT gear in Manual (to eliminate preflashes), or by stacking a trigger with TTL passthrough between your on-camera RT transmitter and the camera hotshoe, but if you want to use RT only, then the choices are limited.
In terms of Yongnuo's (cheaper) RT-compatible gear, they've made a 600EX-RT clone, the YN600EX-RT. As it was only released late in 2014, there aren't many user reports yet. Going by Amazon reviews of the unit, however, it sounds as if there are a few teething issues. This is typical of new Yongnuo products, where early adopters tend to be inadvertent beta testers. Learn about firmware updates and manufacturing date codes if you want to go down this rabbit hole.
Yongnuo also has their own clone of the ST-E3-RT, the YN-E3-RT. The main complaints on this one are that the battery continues to run down even when the unit is turned off. However, it does have an AF-assist which the Canon version does not, and will allow for some of the RT-post-2012 features on remote flashes with pre-2012 bodies; again something the Canon unit does not.
In addition to this, Yongnuo also recently announced that they're working on what they're calling the YNE3-RX receiver, which should allow for integration of non-RT lights into the RT triggering system. To quote from the Flash Havoc article:
YNE3-RX Wireless Flash Receiver
- Supports Flash modes: ETTL, M, MULTI
- Supports single contact flash trigger
- Supports remote shutter function, test firing
- Supports PC output interface
- Supports USB upgrades
- Supports single contact lead flashing
Compatible transmitter (master unit):
- YONGNUO – YN-E3-RT, YN600EX-RT,
- CANON – ST-E3-RT, 600EX-RT
Compatible ETTL flash list
- CANON – 600EX-RT, 580EX II, 430EX II, 320EX, 270EX II
- YONGNUO – YN-568EX II, YN-565C, YN-468C II, YN-467C II, YN-465C
But as most of the promotional text is in Chinese, clarifications on these points is unlikely to be forthcoming until reviewers have actual units in hand. But if these work as stated, this could be the "missing link" in the RT system that Canon should have provided.