I don't know if Axl himself ever mentioned a trilogy of albums. Seems the plan for a while was to release two albums worth of material from the sessions.
Which songs ended up on the first album and which were saved for the second album seems to have been largely shaped by what had leaked. If Best Buy was paying you guaranteed money for the first album, it would make sense to unload as many of the already leaked songs as possible.
If Eddie Trunk had played Perhaps on the radio in 2005, I'm sure it would have been put on the first album instead of being saved.