It’s not fair to split it 50/25/25 as you’ll be having an equal share of the other facilities.
40/30/30 is much fairer.
Agreed. If it were two rooms in a hotel, 50/25/25 would be fine, but not if it's a whole apartment - it's not fair to expect her to pay half of the cost of the living room, kitchen and bathroom(s), which will presumably be used equally.
If you do expect her to pay 50% of the whole cost, be fully prepared for her to expect the same proportions when it comes to time spent in the bathroom and kitchen (or the decision on what meals you eat, if you're cooking together) and what you watch on the telly etc. She will (not unfairly) see it that she has half the say and you two together have the other half.
Having said that, though, did she outright demand her own room and/or are the two of you naturally much closer to each other than she is to either of you? Did you two offer to share or did she just expect you to? Would it have been better to find a 3-bedroom apartment, or did you really not mind sharing whereas she did?
Obviously, it's not the case here, but if there had been a lovely huge, light double room and a tiny little box room and either you two had instantly seized on the bigger one - or it had been obvious from the length/closeness of your relative relationships that you two were the natural choice for sharing, a 33/33/33 split would have been the fairest way.
I wonder what her reaction would be if either or both of you said that you also wanted a chance to have your own room and suggested drawing lots for it (with the 'winner' paying the greater share) - would she accept that?
With a lot of these accommodation-sharing scenarios, there's often a lot of deliberate blurring between what certain people need as opposed to what they want or feel that they deserve (which is often not dissimilar to what everybody else would want too). "I'll take the much bigger slice, because I really love carrot cake" - yes, because everybody else hates cake, don't they....