Aces are good for suit contracts because they represent "first round" control. In a trump contract, there might only be one or two rounds of a side suit played before trumps are used to control the suit.
If you are playing it a major suit trump contract, declarer and dummy will likely have a mid-twenties (Work) point count. That would average to 2.5 aces, pro rata.
As a practical matter, most such hands will have either two aces (below average) between them or three aces (above average). A few will have one or four.
Studies have been done that show that the chances of making a major suit contract are positively correlated with rising point counts, with 25 points being the closest to "50-50."
Have these or other studies shown that hands with three aces are more likely to make than hands with two aces (holding point count constant at 25, 24, 26, or any other number)?