A pair of 3's is played just like a pair of 7's, and for similar reasons.
Split against a dealer 2 through 7, otherwise take a hit.
A total of 6 is going to likely wind up as a total of 16. (We're assuming we'll receive a ten.) That's a tough hand, but against a dealer 8 or higher, we're in trouble anyway, so we might as well NOT put more money in the pot.
Against a dealer hand of 12 through 17 (again, assuming that we've got a 10 in the hole), we've got a pretty good shot at either getting a dealer bust, or in the case of the 17, outdrawing the dealer.