RE: when do you know you are ready to go to third plane?
Greetings from South Carolina
Have someone that is comfortable to take you up with this plane, see if you can find a comfortable height, then find out what the stall characteristics are at this height, pratice landing approaches and keep in mind that the further you get away from trainers the faster the landing speeds need to be in order to keep from stalling this airplane.
It is all about maintaining control of your aircraft, you already have the tools in place to determine the pattern and approach needs to be in order to take off and land a trainer, once you move away from a trainer, you have to learn what will happen when this new airplane stalls, then keep the speed above this speed.
I hope this is of some help, keep up the pratice.
Tom