EDITED: Found it. You don't import db3...you open them.
I have some older PGO db3 from 2013...made with the prior version of PGOffline. I am having trouble importing them. When group is selected it shows 180 messages but nothing is imported.
Technically you should be able to import them but as you've noticed there's probably some subtle difference.
I would think that you could export as PG4 from the db3 you have and that could be imported into a more modern db3.
Cheers,
Wilson.