Like mattsbettas said, they do have a very low bioload, you could do as many as 5 or so in a 3 if you keep up with a water change schedule like mentioned in OFL's sticky (1 50% and 1 100% per week) but you need to make sure that there will be tons of hiding spaces for them because more than likely atleast a few will become a snack for your boy.
I would add a ghost shrimp first and see how your betta reacts to that before trying something as expensive as cherries, but even then it's still not a good tell-tale since cherries are so brightly colored compared to the ghosts that they'd practically be a bullseye for your betta
When you change your betta always acclimate him to the new water temperature. You can use the container you bought him in with about 15% of the water(just for the short period of time). Keep adding small amount of the water about every 5 min and do it for about 5 times, this way he will get used to the new water temp and you will not stress him with drastic temp change.
Also always cover container and a net that you are using to couch your betta so he don't jump out. They are really good jumpers. And don't forget to use water conditioner every time you change him. Leave space between the tap of the tank and water level.