PREFLOP: Yes you are right, you should fold. I usually will only open raise with QJo in the cutoff or button sometimes in the hijack also if the players behind me are tight. Occasionally I will Isolate someone with this hand when im on the button or the cutoff, but this depends on my read of my opponent and again on how loose and aggressive the remaining players are. In MP1, you should fold IMO.

FLOP: Checking and calling looks good to me, given the size of the pot, you must draw to your gutshot even though there is a flushdraw on the board.

TURN: Ok you turned the straight, but the third diamond hit...now what? Usually when I turn a strong hand and the flop aggressor is directly on my left, I would go for the checkraise, however since the 3rd diamond hit, your hand isnt that strong, usually I would bet the turn myself and call a raise(unless I had a read that my opponent was raising without the flush).

On this board however I think your check 3 betting strategy may actually be the best course of action since even though there is 3 diamonds on the board, I cant see how the preflop 3 bettor can have a flush or how the button who coldcalled 3 bets preflop can have the flush. Hands like QdJd, Jd9d, or 9d8d should not be there if these opponents are playing somewhat sane, so you should have the best hand here.