Settle this No-Limit dispute

[LarsVegas]

Live-game, blinds $30 and $30. UTG and "Z" in this hand are both fairly thinking, yet not expert players and a little bit on the loose/gambling side.

UTG limps, another player limps, "Z" limps, I check 44 from the small blind, big blind makes it $150 total, UTG limps, player in the middle folds, "Z" calls and I make the call of an additional $120 too (very close or even wrong, I know).

Anyway, flop comes 6-5-4 rainbow. I check, big blind checks, UTG bets $300 and "Z" calls. I am not particularly worried about the big blind, and he is only playing $1200. I am playing $3800 and both other players cover.

What is my play and are there any close lines?

best wishes

Lars