I noticed that the zebra and horse only walk or gallop whereas the wild ass only walks or trots.
Isn't there a way that all 3 species can have all 3 gaits?
Yes, I noticed that too! Funny! Of course, they all should have all three gaits. However, it is true that wild horses rarely trot. Most of the time, they walk/graze and only throw themselves into a gallop when they feel in danger. Trot is in between and may be used to catch up with the herd if necessary but is less common to see in the wild herds.
As for the wild ass, I'm not sure why they wouldn't sometimes canter or gallop. Donkeys are not as easily scared as horses and will more likely kick back instead of running from danger. Maybe this is a reference to that? Or the game designers just wanted to make the animations more diverse for different species. But I would be glad to see all three basic gaits done for all equines.