The leader depends on your starter. They brothers and have three different Pokemon. Who you fight depends on your starter. If you choose the fire starter, you fight the leader with the water Pokemon. If you choose the grass starter, you fight the leader with the fire Pokemon etc.I'm not seeing why a gym needs more than one leader.
It makes it harder. The gym leader you get has a super effective type. If it was the last badge, chances are that you've caught a Pokemon that beats fire or whatever you got easily.having those three gym leaders would be a great idea if it was for the last badge..
having them too early really makes it more irrelevant since it would be kinda easy :S