Wait atleast a week or two, just check your readings (ammonia, nitrite and nitrate) and make sure they're fine, before adding fish. What do you plan on keeping in this tank? And got any pictures?
Erm, well you can't really say that, it depends on the size of the fish, 3 more fish could be 3 large cichlids, or 3 small neons (which in that case they could have more).
But you understand what I am saying, being that cherry barbs are about 2 inches. And 3 times 2 is 6 gallons so there are for galons left of room and 3 1 inch fish would leave some breathing room for everybody else.