About twenty hands into the table, with no significant out of line actions on any side. I'm running very true to my standard ABC 14/11/3 style. (Was on doom run, so converted back to ABC and dropped stakes)

UTG+1, 14/8/3 is a positionally aware, tight, ABC style reg over 1800 hands this is his second flop check raise.

MP2, 10/6 over 400 hands. Nitty like he ain't got manners

CO is a 2 hand unknown.

PokerStars Pot-Limit Hold'em, $0.50 BB (8 handed) - Poker-Stars Converter Tool from FlopTurnRiver.com

saw flop

Hero (MP1) ($50)
MP2 ($74.45)
CO ($22.05)
Button ($11.55)
SB ($12.75)
BB ($88.60)
UTG ($48)
UTG+1 ($52.25)

Preflop: Hero is MP1 with 3, 3
1 fold, UTG+1 bets $1.75, Hero calls $1.75, MP2 calls $1.75, CO calls $1.75, 3 folds

Flop: ($7.75) 6, 9, 3 (4 players)
UTG+1 checks, Hero bets $5.50, MP2 calls $5.50, 1 fold, UTG+1 raises to $18, Hero ???