It's one of those situations where people will never agree and just have to agree to disagree.
Some people don't understand the fact the rent of the singleton is subsidised owing ot the fact it's a 2 bed house, and others can't accept that splitting the cost 3 ways is "fair". There are so many factors to take into consideration that it's impossible to gauge on the information given... i.e. how much more space the 2 people take, how much space the 1 person takes, how much equipment/furniture they have, shift times, eating habits, being frugal with heating and electricity etc...
Personally I think that if it was a 3 bed place with 3 people but 2 sharing one room then 1/3rd share would be fair, but in such an instance it's not. The singleton is getting a cracking deal! If it was just 2 people in there the singleton would be paying a significantly higher rent. So the way it's been split to me is fair.
Each to their own though, it's what is agreed between people. Some people take into consideration living space, and social habits and living habits, others just do it purely on numbers vs people.
If the OP and his friends are happy with that, as long as they get it agreed and signed by all 3 in writing - then there are no issues in my eyes.