Personally, I don't know any kind way of asking someone to pay more for rent than me when I already pay 200 less than my only other roommate.

OP's problem is that he doesn't like his living situation. Like someone else said, this is a roommate problem and not a financial one. Even if they spontaneously started chipping in extra without his prompting, he'd still find the girlfriends presence bothersome because he's living with 3 people not 2 which is something he didn't sign up for.

OP already mentioned he wants to buy a house next year, should probably just buckle down and continue saving versus risk the awkwardness that can come when you ask for more money from one person and they say no or it turns into a fight over who left dishes in the sink 3 months ago, etc.