A small sledge will open any of them
but that is on the drastic side.
You're trying about everything that works. Prevention is the main thing.
First, if the paint leaked out when it tipped over, the lid was definitely too loose, and if it doesn't get welded shut, it's going to dry up the paint over time anyway.
Someone else mentioned the main thing. Wipe the threads on the jar, and wipe the threads in the lid and where the top edge of the jar hits the gasket in the lid. Then make sure the lid is closed tightly. If it can't leak out, it can't seal the lid shut after that. Yeah, it wastes a little paint each time you do it, but you won't waste the whole jar either.
Good luck getting those open. Once you do get them open, clean the treads on the jar and the lid before you close it back up.
edit: Try some laquer thinner in the lip of the one that's still stuck. That's the strongest of the "normal" solvents for paint.
Eric