Definitely not impossible, but expensive as hell.
Even as easy as it is to get a hold of a 3 rotor engine to install it in a 2nd gen rx7 like I have doing the labor yourself will cost between $10k and $15k. There are a few places around the country that will do it for you for about $30-$35k. I assume the price for the 3rds gens would be similar for the most part.
So considering that just buying a 4 rotor engine would surpass those prices likely..... ouch. I'll stick with my 2 rotor engine, she'll have more power than I know what to do with pretty quick here. Just got my walbro and 4 865cc injectors, hopefully be getting a new turbo and megasquirt ECU in next summer.