system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: A couple renting a two bedroom with an individual should split the rent three ways.

The argument against seems to come down to the fact that the couple considers themselves as one entity while the individual sees them as two separate people.

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You can argue that you're paying for 50% of the bedrooms (true but that's your personal decision) and 50% of the living space (wildly inaccurate as no matter how you slice it two people take up the same amount of space whether they're in a relationship or not--everyone is getting 33%), but the reasonable approach should be to split everything evenly. Realistically, the individual isn't getting the living room twice as much. Not getting twice as much refrigerator space. Not having people over twice as much. Not getting twice as much access to the apartment's amenities. Not getting to put up twice as many decorations. Not getting twice as much voting power when it comes to decisions or establishing house rules. That would be the only way a 50-25-25 even comes close to being fair. The couple is putting all the weight on the bedroom as if that's the only significant feature of the apartment and as if they wouldn't be sharing one, by choice, anyway.

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If these people were splitting a two-bedroom hotel room, would they expect the individual to pay twice as much just because they physically share a bed? That sounds ridiculous.

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Three adults. Three incomes. Doesn't matter if your friends or complete strangers. You shouldn't even have to break out the tape measure to try and break it down to the square footage. The *reasonable* decision is for everyone to pay an equal share. This is especially true in a high cost-of-living area where a greater portion of your income is going to rent. If you're fine with one person struggling while you--the couple--has room to save/has more disposable income based upon something that is entirely within your control, you're kind of being shitty people. You already have the advantage of sharing your incomes. You don't need an additional discount on top of that and especially not one at the expense of the other person.
gpt
The truly fair way to do it is cost per square foot. 

Total rent / total square footage = cost per square foot. 

Living room, shared bathrooms, kitchen, etc - all common spaces - square footage cost is divided evenly between the three roommates. This isn't about who uses the space more, but that all three have equal rights to the space regardless of use. 

Bedrooms and private bath square footage are paid by those who occupy them. Single roommate pays full cost of their bedroom square footage, couple takes care of the square footage of of their own space. 

Depending on apartment layout the payment ratio changes, but ultimately everyone pays specifically and proportionally for the spaces they have rights to. 

Of course, despite being the fairest, this is a tedious way to divy up the rent; so as others have pointed out here, what's the issue if a different arrangement is agreed upon?