My rule for 3b isoing is if I think I'm ahead then I 3b....
This is a pretty good basic rule for Btn vs CO type situations, but you have to tighten up the more players you have behind you left to act. For example, I think its a mistake iso a hand like A9o in EP even if villain is opening like 80% of hands from that position.
In your first example, I probably don't even open 55 in EP if its folded to me, so I don't really like iso-ing with it. Yes, you have more fold equity, but you still have to respect the chance someone behind you wakes up with a hand and 55 doesn't play well 3-way for 4-bets preflop.