Yah, 10 gallons is definitely the best. One of the reasons why 10 gallons is considered the minimum ( no matter how many or the species ) is because the less water you have the more often you will have to do water changes and the easier it is for bacterial levels to fluctuate. Which is not good.
However, it does depend on the species somewhat. You probably aren't going to want more than one P. waltl in a 10 gallon tank. On the other hand you could have maybe 3 C. orientalis in a 10 gallon tank. So it kind of depends on what species would go in the tank, and if the tank is a breeder or a tall etc. And also how planted it is.
But like Jake said, 3 gallons is simply not large enough. -Seth