Well I don't really understand this as much as everyone else. I'm going to have to learn it by trial and error I guess. Because honestly I don't know why it would be a bad thing.
But if we were playing the trading card game, I'd take you ALL to school :emoji_slight_smile:
The issue when it comes to sharing too many mons of the same type means they could share too many weaknesses.
The only mons I currently have on my team who share a type is Rotom Wash and Volcanion as water types and they do not share weaknesses. Rotom Wash is only weak to grass (it would be also weak to ground but Levitate removes it) but Volvanion's fire typing removes a grass weakness. Not just that but they play slightly differently. On the bright side for you is all three Pokemon are good (*hugs the real Noodles*) just all three are weak to other dragon types, two are weak to use, and two are weak to rock. So that is something you need to work around.
It isn't to say you can never have overlapping types, even if they share weaknesses, it's about mixing your team up to balance it. But hey, we all learn and I'll give you a big lesson week nine. :emoji_blush: