The main difference is in the damping, both have similar other options like Dual Position and Solo Air, length etc.
With RC it's more like a 'progressive' dial. From full open indexed as you dial to locked. This affects the 'stiffness' of the fork. In other words when riding uphill, you dial to 'lock', it firms up and you have less front end bob. Whilst it helps a great deal, the fork is not completely 'locked' as you would expect and the dial is more targeted at low speed compression. It does however firm up quite a bit.
With RCT3 you have presets for the 'stiffness' and a low speed compression dial. With this setup you get more of the actual lock-out from the O/P/L(Open, Pedal,Lockout) dial and the low-speed adjustment (the dial for RC) is now a 'fine-tune' dial on top of the O/P/L dial. This system acts more like the actual meaning of the words, whereas the RC 'emulates' more.
Make no mistake, even the RC is butter smooth. As far as simplicity goes for feel, the RC Solo Air is a very good fork. Add air by rider wait, dial rebound according to terrain and just go smash it.
No offence, but if you had to ask what the difference was between RC and RCT3, I would suggest that the additional 'features' on the RCT3 wouldn't make a difference for you in regard to ride experience - they are more for advance/experienced riders really.
Whilst it is somewhat poorly done, you can have a look at the SRAM specs for detail.