The rent gets split 2 ways – you pay 50%, they as a unit pay 50%. The bills get split 3 ways, each paying 33%. You are each paying for living in a room of the house. You are getting one full room. They are each getting half a room. That’s the rent. The bills, however, cover things that the two of them cannot consolidate. You’re all using one third of the electricity, the water, the heat, the cable, the internet, etc. (I don’t see the dog factoring into the equation at all)