Original question?
when do you know you are ready to go to third plane?

Answer:
When ever you have crashed the previous two planes.

I have flown the shoestring plane on my 4.5 sim, even that we all know that it is way different than the real feeling, I can tell that this one will keep your sticks very busy, and needs to come hot on landigns because it is easier to stall than any other sport plane. It is a really nice plane but I think that it is in the limbo, I mean it is not exactly manufactured by great planes as a racer performer and it is not a reliable sport plane, but hey that is my opinion.