I'm having this same problem as the OP. I either can't find or obtain a S/N for Amplitube 3, It does not show up in my ik account, and a bunch of styles are linked to it and it's presets. I can bridge Amplitube 4 as a 32 bit plug. But still - to me this is an unresolved issue. I should be able to authorize and load Amplitube 3. I can do neither and have spent the better part of a day trying.

Currently waiting for a response from PG.