No, player C will keep controlling it.
Both spells have continuous effects which modify the creature’s characteristics in Layer 2, and the one with the latest timestamp wins. It doesn’t matter that Act of Treason is only temporary (and one could say Donate doesn’t have a duration at all):
611.2a A continuous effect generated by the resolution of a spell or ability lasts as long as stated by the spell or ability creating it (such as “until end of turn”). If no duration is stated, it lasts until the end of the game.
Relevant parts of the layer system:
613.1b Layer 2: Control-changing effects are applied.
613.3. Within layers 2–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.7). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.8.)
The situation would have been different if Act of Treason had read something like
Gain control of target creature. At the end of turn, return it to its owner’s control.
because then that last part would be ‘later’ than, and overriding the effect of, your Donate.
A related example: you cast Donate on one of your creatures and give it to Player C. Player A casts Act of Treason so it comes under Player A’s control; at the end of turn, it’s returned to Player C, not to you.