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Suppose:

Power 1 has armies on squares V and W
Power 2 has an army on square X which is adjacent to all other squares
Power 3 has armies on squares Y and Z

Orders
Power 1 attacks power 2 (square X) with support
Power 3 attacks power 2 (square X) from Y with support from Z
Power 2 enters a move order X->Z

Does the X->Z move kill support now two powers attack at the same time, or is it still a bounce?

I believe Power 2's order X-Z does NOT kill Power 3's Z's support for Y-X, because of the "except" part of the rule:
"Support is cut if the unit giving support is attacked from any province except the one where support is being given."
Hence it's a bounce between Powers 1 & 3, so X stays put.

I agree with echidna's analysis. Everyone would bounce in this scenario because Powers 1 and 3 bounce in square X and Power 2 does not cut support on an attacking unit.

As the others pointed out, it is a bounce between powers 1 and 3, since the support that would have been cut otherwise is not cut, since the unit that would be cutting it is the unit being attacked.