I have a question about the decision that most people think you should have made pre-flop: 3-bet. Do people recommend this just because of the limit? Would people play at higher limits, say at 5/10?

I agree that it's the correct play here, since we can't limit the hands of MP3 or CO.

But at higher limits, the hands that seem most likely for our opponents are overcards (perhaps an overpair). If we give them a total of 3 overcards (for the times MP3 or CO plays with AT, KT and other random hands), then they'll hit one their cards ~46 % of the time on the flop. Since we don't have position, it makes it more difficult for us to tell when they've hit and missed, or when they've got an overpair. At a higher limit, I would err against the 3-bet.

Is this thinking right? Am I too passive?

And, if you wouldn't three-bet, what would you do? Call, a scope the scene on the turn?