I think Dano was absolutely right (like always
) at the end of you other thread. I shoot three under and use primitive self bows and pyramid board bows exclusively, neither of which are center shot. I spend a lot of time trying to get proper arrow flight. Yet before I start trying to "fix the bow," I consider these other aspects first (assuming tiller is correct on the bow, which seems to be true in your case.)
1) Arrow Shaft Selection: You can't rely exclusively on a chart to prescribe the arrow you should be using. There are just too many factors at work. Guys who shoot non-center shot bows will be quick to tell you that spine weight, shaft weight, and arrow length are often a matter of trial and error. Now, a center shot bow will obviously be more tolerant of variances in spine weight and arrow length, but if in fact you've got a mismatch between your bow and arrows, get yourself a test kit from 3riversarchery.com or somesuch. The kits are cheap and will give you a range of shafting to try.
2) Brace Height: Again, I find this to be more of an issue on non-center shot bows, but there is a sweet spot here. Many will agree that a slightly higher brace height is needed when shooting three under, as you are.
3) Bow-Hand Grip: This hand can do funny things to the bow, and in turn the arrow, if not kept in check. Are you torquing this hand upon release? Are you gripping the bow too tightly? We could go on and on. Be critical here, and search the shooter's forum for some tips if you think this may be the culprit.
4) Arrow Fletching: For me, discovering the correct fletching configuration has been critical in achieving stable arrow flight, even from gnarly old stick bows. (In fact, I built my first character bow not too long ago and learned just as much about tuning an arrow than I did about working a funky stave.) How long are your fletches? Are they applied with a helix? How far back or forward on the shaft are they? Have you considered using longer fletches or going to a four-fletch design?
5) Cock Feather Placement: I discovered awhile ago that for me, I get better arrow performance when I place the cock feather in. Simple, cheap, and easy fix that puts the arrow on the mark and keeps the fletches in good shape. Maybe that could help.
6) Release: Do you shoot with a tab, glove, or fingers? Do you use a deep or shallow hook? Are you torquing and/or plucking the string upon release? Are you getting a little bit of creep just before full draw? Are you transferring the weight of the draw to your back muscles? The questions could go on and on.
7) Nock Point: Don't be afraid to really experiment here, especially if shooting three under.
So many easy and cheap variables to consider before making tool marks on your bow. Good luck!