AFAIK the 3SGTE flange only fits the CT26 and only for its particular model
i.e. Gen1 and Gen2 3SGTE's have slightly different CT26 flanges.
Gen 3 has some form of CT20 / CT26 hybrid jobby.
1st Gen will fit with a bit of modding but why would you want one
The Gen III (commonly refered to as the CT20B) is a completely new turbo and has nothing in common with the early CT20. It's a straight bolt on to the Gen II but apart from flange bolt pattern and external comp housing dimensions is a completly new turbo to the CT26.