I've recently started shooting three fingers under and love it. Now I'm build a new r/d longbow and want to see if this makes sense to you guys.
I have a Tomahawk SS, the shelf is cut 1 1/2" above the center of the bow and tillered 3/16" and it shoots loud 3 under.
I have a r/d of my own making, the shelf cut 2" above center and tillered 1/8" and it shoots pretty quiet 3 under.
I have a kimsha r/d the shelf w/leather rest is 2 3/8" above center of the bow and it is tillered 1/8" and it is very quiet.
    I can see where this is leading in building my new bow but can anyone explain why?
    What else should I be thinking about when building for three under?

Thanks in advance. Brad

I think basically three fingers under requires a more negative tiller or less positive lets say.

Like Dano said- I think it is pretty common to tiller about even for three finger under shooters.  1/8 to 3/16 is pretty standard for two under shooters.

I am shooting a Bear R/D and I shoot it 1 over and 1 under, 1 over and 2 under, 3 under and 2 under. And those that shoot next to say no matter what I do the bow is quiet. Now this dose effect where the bow shoots. It seems that that when I shoot with one over and 1 or 2 under arrow flight seems to be in a cork screw, 2 under seems to be the best flight 3 seems to give me a shot that drops about 5 inches @ 20 yards. Maybe I should look into this cutting the riser and tillering for finger placement.

Thanks all for the help, I think I"ll start w/ a high shelf and an even tiller and see what that gets me.

Makes sense since 2-3 under places the draw force lower on the string and it would stress the lower limb considerably more than split finger. Moving the shelf up in a way lengthens the lower limb and should help keep it in time with the top limb. Keep us posted on your results.
Thanks

http://www.tap46home.plus.com/mechanics/

This is the best discussion I have found, although he keeps moving his site around.  I've used it for many years, and have yet to find exception to his thinking.  To be fair, he generally sumarrizes and builds upon work published by Kooi.

The section titled Bow Mechanics - Tiller dicusses what you are asking, but the whole dang site is, I would argue, prerequisite reading for those interest in bowyery and tuning.

The rule of thumb I follow is, farther above dimensional center, more positive tiller.  Since three under tends to put the forces closer to center the "even tiller for 3 under" rule most bowyers will quote seems to follow.  But it's not just a simple, single element.  The tiller shape matters too, and nock point obviously can be used to tune out or in more or less tiller.

After making dozens of bows, I can't honestly say I've found a method or formula, other than trial and error, that works in all cases.  But the general rules of thumb do seem to apply.

I have been making mine even tillered with good results.  The only change I will make in the future is I want to make a bow with 1-1.5" longer top limb and then with 1/4" positive tiller.