There is an optimal bluff rate from GTO (Game Theory Optimization). You bluff at a rate that call versus fold has the same outcome for your opponent. Set that EV (Expected Value) to zero and solve for the bluff rate.
pot = pot size at start of river
bet = bet as a fraction of the pot size
bluff = fraction of bluffs
EV = 0 = -bet + bluff(pot + 2bet)
bluff = bet / (pot + 2bet)
set bet = 1 (pot)
set pot = 1
bluff = 1 / 3
so for every 2 winning hand bluff 1
if they call all three the EV is 0 = -1 -1 -1 +3
if they fold all three EV = 0 + 0 + 0
It is fair to call it a raise but it would be more proper to call it open for $3 on the flop. With $3.75 in the pot that is almost a pot size open. That is a strong call. They have a piece of it (like a Q) or a strong draw. Since you have the flush covered including the Ace maybe JT for an 8 outer straight draw.
Turn fire again with an almost pot size bet. You have a flush draw and two over. You get another call. That is a very strong call. That does not fill in a draw so now you need to put them on a made hand like one pair, two pair, or even a set. JT is still a possibility.
River hits JT for a straight and 8.
Need to sell that you have a winner. Would you have played JT, Q8, 98, QQ, 99 that way? Get AQ or KQ to fold out. Or they could be on a busted draw. Your risk is here is they could be on 2 pair or better and not going to fold.
I think a better plan is to check the turn. If they bet big just get away. If they check bet the river no matter what. You have just as much fold equity with 1/3 the investment.