I can only answer your question about 3 (Since I've only played that and Reborn). If I remember correctly, the big thing with mobile suits in that game is that they level up through blueprints.
I don't fully remember the details, but (if I'm correct) you have to clear a specific mission to start getting rank 3 and 4 plans (The most powerful ones). I think that they're also random droping, but you're more likely to get them if you play as that unit.
Sorry to double post but, the reason I need to confirm from all of you on that I'm using Lacus Clyne from Gundam Seed and I let her use Heero's Wing Zero to start with and if she reached max level 50 and it's complete leveling I'd have her use Domon's Burning Gundam to see that it will increase in Chain Combo and SP Gauge is because.
I'm worried about Amuro and his mobile suits and this is History Missions that I'm talking about and the one is he is to use the 1st Gundam in the 1st anime in that mission.
I haven't been replying because it's been God Gundam-knows how long since I played 3 and what I told you is all I remember about it...wait...I think the SP gauge is pilot-based in 3...I don't fully remember.