A new main release is considered as a new product, infact it is, both ArchiTerra 2 and 3 can run inside the same ArchiCAD as two different Plug-ins, so there's no Compatibility Update.
Cigraph only give a 3 months free update period for those who recently bought a new ArchiTerra 2.
For more info:
http://www.cigraph.it/cigraph/pagetrans ... news_id=64
thank you.