1 Rosy Barb?

Rosy barbs are one of the larger barb fish. They are schooling fish and their schooling number is at least 5, not 3. I would return him and explain the situation to the store and then get a male betta. The fish will do nothing but become stressed and die and you'll have to watch it, not able to do anything at all except for get a bigger tank and more of the fish.

NEVER ever listen to the people at the pet store. They stock based off the "inch per gallon" rule and do not take anything else into account. Even when they seem to know what they are talking about, take their words with a pinch of salt and do not trust them.

Fish can't live in a tank that small, it's even considered a little too small for bettas. That rosy barb will soon die from stress because he's not in a school. That doesn't mean to go buy 4+ more barbs for your tank, because if you do they will die from bad water parameters in a overstocked tank. Rehome him to someone on Craigslist or take it back to the store for store refund, or most will just take the fish but won't give you credit.

Did you notice the adult size of a Rosy? It's bigger than your tank.