Wrong.The rule of the Striaton City Gym is that there must be three Gym Leaders present so that challengers can choose which elemental monkey they're going to battle against.
The rules of a gym are made by the gym leader. Because there were 3 they had to let the challenger choose which one to battle.
But this was just "their" rule. But there's no reason why there couldn't be less gym leaders, it would simply mean less options for challengers.