If you shoot 3 under, which limb-Top or bottom should be backed out from bottom to compensate for shooting 3 under? Thanks in advance

I would start with an even tiller.

I agree.  I haven't seen any solid evidence that tillering specifically for 3 under makes any real difference.  It might for a Rod Jenkins - Ricky Welch caliber shooter, but I haven't seen it.

I shoot split and my upper limb is almost 1/8""-3/16" longer than the bottom but I can't remember how many turns that is on the top bolt. I'd back the top bolt out a turn and measure and shoot until you get it right.

i have yet to see any difference...the only difference it makes from what i've seen is it will make a noisier bow when shooting three-under

I have had mine for a while now. And this is what I  have come up with. Go to the hard ware store get some plastic washers and fix the tiller problem that way. So when you take it apart you don't lose the tiller. I screwed mine all the way down and took a measurement and had 3/8" tiller on the top limb I also shoot 3 under. I also have a 1/2" high nock point. Mike