From what I've seen, If you can find 3 power stones with the EXACT same stats, (Given that you like the 3 stats) the stats from the first stone, will cancel out the identical stats from the 2nd and 3rd power stone, leaving you with simply the 3 stats on the first one.
OK, I'm quite novice at AT, but could'nt that imply that to add the fusion stone's green stats you want on the final item, you only need one fusion stone with the wanted green stats and two other fusion stones with the same green stats (not the green stats you want, can be any type of green stat as long as they are the same on each fusion stone BUT different from the wanted green stats you have on the third fusion stone).
Indeed, you would have the green stats you don't want cancelling each others in the transmut process, leaving only the remaining green stats of the third fusion stone, the ones you wanted, to be added to the final item.
Given the number of people who use AT, I guess it would have been discovered by now if it really worked like this, but I still keep a slight hope of this beeing true.
That would simplify a lot the process of fusion stones matching (easier to find two matched fusion stones with undesirable green stat + one with wanted green stats than three fusion stones with matched desired stats...)