if you bet on fifth with nothing against that many loose players....

I'd raise sixth because that (appeared) straight isn't going anywhere. If the two pair guy has one of the only two cards in the hole (one 3 is dead and that other guy is representing a 5 in his hand) [Also I don't think he has three 3's] I'd be surprised. I doubt the straight is folding so I'd raise--it's way easier getting an extra bet from someone who has any outs and might suckout on you.

Or just call and raise the river (but I can't see two pair calling that)

Here's raise scenario
straight extra bet

call scenario raise river
two extra bets potentially (from the two pair guy and one from the straight guy on the river)

I don't know how often that the latter will happen unless I have at least a decent read on that player.

edit--I think the two pair guy has three pair and I would want that to pay to catch.