A three-gallon aquarium can support three 1-inch fish. The basic rule of thumb when adding fish to an aquarium is that each 1-inch fish needs one gallon of water to give it plenty of room to swim and grow.
If your choice of fish is a small species, then this rule should work fine; however, larger species can easily outgrow a tank, forcing you to take action down the road. When calculating how many fish to add, consider how much space any decoration will consume; this could turn the space in a three-gallon aquarium into a two gallon area. The more fish you have, the more filtration and maintenance will be needed.