Bear in mind it is a 90 ELPTO, it's never going to idle like a sewing machine. Well adjusted the idle will be reasonable to good at best.
My manual for the same engine states 1-1/2 turns from lightly seated as a starting point on all 3, it states this both in the timing section and in the carb section.
All 3 carbs are essentially the same so having the same setting would make the most sense to me.
I would double check all the carbs are opening together and by the same amount. You can use a drill shank (about 1/8") as a feeler between the carb throat and butterfly valve, all 3 carbs should feel the same. Just don't drop the drill into the engine.
I would crank up the idle speed, say to 700 rpm when the boat is in the water and in forward gear. Should result in the engine running about 800 rpm when in Neutral. You could go slightly higher as long as you can still shift gears smoothly.
Do the low speed mixture adjustment as stated in the manual, then reset the idle speed as above.
New plugs can help too.
