all three factions are enemies to each other
There is also an optional rule 21.0 in the C3i supplement (see files section), which allows Soviet and Western factions to win independently.
In that rule the Allies still need to conquer Germany to be eligible for victory. I suppose it could still in theory lead to something of a temporary alliance between Axis and one of the Allies, if the Axis believes it can save itself by that move, and the Allied faction believes it can still conquer Germany in time but gain more points.
Maybe you could start from there to build something that gives better incentives for one of the Allies to switch sides?