Nor'easter and Falk nailed it. AMO states that the AMO bow length is 3" longer than the string length at brace. String length is to be measured at 100# tension for everything except 10 strand dacron strings, which are to be measured under 50# of tension.
However, that doesn't amount to a hill of beans if the bowyer/manufacturer doesn't follow AMO specs, or if the archer prefers to tune their bow outside of the specs. Very few string makers follow AMO specs either--especially the measureing under tension part.
If 3 Rivers doesn't pre-stretch their strings, you'll probably want one 4" shorter than the AMO, because it's going to settle in quite a bit and that should allow you some room for twisting.
Chad