Completing pre and checking flop are fine, but firing the turn multiway with total air is too loose; pick bluffs with equity or better blockers.
Three-way on Jd Ks 9d with 8-high and no real backdoor value, checking is automatic. This board favors the overcaller and limper; we have range disadvantage and no equity, so building a pot out of position with a stab here would be torching chips.
Turn Ac is a big card for both UTG’s limp range and BB’s range, and we still have pure 8-high with no draw; multiway, this is one of the worst bluff candidates. If we’re going to attack a check-through, we’d rather use hands with gutters, flush draws, or cards that block Ax/Kx. Betting 3 into 3.5 will work sometimes vs overfolders, but in theory this combo should just be checked and given up.
Note: The turn stab is too loose: 8-high with no equity or key blockers multiway on a very favorable card for Villain ranges should mostly check-fold.