For a start, T3 is newer and is not just a terminal. It is a integrated shopping, retails (in and outside of immigration) structure with better connections/access to the subway station, Jewel, airport hotel and App pick-up services.
In addtion, for those in Polaris cabin, you are access the brand new SQ Sillverkris lounge. I am a PPS member, so I can access Silverkris lounge when flying Y. I have no idea what *G Y pasengers use at T3. the new Silverkris Lounge is quite nice, and will be further expanded.

SIN does not have a dedicated *A terminal. Most of the *A partners are flying out of T1 which will likely going through renovation after T2's current renovation is completed.