My 10'6" Nalu surfed best with the standard fin set up (with the side fins in) and when I took the side fins out the tail felt a bit drifty and had less drive on bottom turns.. and even tracking and stability was slightly reduced.. I then tried a slightly bigger centre fin and that fixed it.. and that's what I use now.. It maybe doesn't turn as well as it did with the smaller center fin and the side fins in but stability and tracking is better and the drifty feeling has gone.
I have an 11 4 and slapped in a quality 10 inch fin .......picks up waves so ridiculously easy compared to my small 8ft 8 sup as you would expect. Not surfed it with sidebites and the stock centre fin yet but that can wait. Good to try different set ups.
Surfed today with the stock centre and sidebites..........waist high slack waves with next to no wind....glassy!
Certainly stick with a decent bigger single fin......much easier to slip into waves and still just as easy to pivot the board onto the waves.
Just felt the two side fins created notable drag......turning maybe smoother but I would keep the two plus one set up for days when the waves have more push and size.
If you do flat water too a fair bit then stick with a single.