Otter, it sounds like you are playing very under-bankrolled for your current levels. Is the $400 all you have online? If so the highest level you should be playing is $25nl (.10/.25 blinds)

There are many Bankroll Management articles on this site it would benefit you greatly to read them.

For the hand, 3betting can be ok in this spot but it does not need to be nearly as big. Your risk vs. reward is pretty awful when you make it $28 to win $4.50 ($3 raise plus the blinds). Instead a raise to $10 or $11 would have to work less than half as much as $28 to be more profitable or less unprofitable.

Post flop you bet $7 into $56, that is a very small bet, one an opponent would more than likely read as weakness. I highly doubt he's going to fold any hand he called for $28 PF with therefore I would just give up on the flop.

Welcome to FTR, please stick around and start playing properly rolled and you'll be just fine.