Yes, you are correct. POA was the first set Artbox released--Emma Watson was the first of the trio to sign for the series. POAU featured the triple auto card of Emma, Dan, and Rupert, as well as single autos of the trio.
This is just a guess, but I think that the trio probably chose not to sign retrospectively and only focus on sets that are released present day, that is with the release of each new film. To my knowledge, there has been no indication that there will be signatures from the trio representing SS and COS. But then with filler sets, who knows--the 3D set released just before OOTP had a Dan auto from GOF, so anything is possible.
In looking at each release the trio has signed for, I noticed that they seem to trade off who signs when, with some exceptions. It looks like Artbox tries to get at least one of them for each set:
Emma signed for: POA, POAU (2x--single/triple), MM, OOTP
Dan signed for: POAU(2x--single/triple), GOFU, 3D, OOTP, OOTPU (dual auto with Katie)
Rupert signed for: POAU(2x--single/triple), GOF, OOTP
Imagine having 4 sets from DH!!!!!!! DH1, DHU1, DH2, DHU2!!!!!!!!

My poor, poor, wallet.

Add to that any additional MM sets...there is a LOT of ground for Artbox to cover with the Harry Potter series for years to come.