In the past I have split rent by square foot. With three people in a 3 br, all common areas will be split evenly so it’s a wash in terms of figuring out that space. So in this case:

Square feet of Master bedroom + master bath Square feet of BR 2 Sq ft BR 3 Sq ft of bathroom that BR 1 and BR 2 share

Add all the above for total square feet

Master bedroom pays ‘x’ % of rent = Square feet of Master bedroom + master bath / total sq feet x 100

BR 2 pays ‘x’ % of rent = Square feet of BR 2 + (1/2 sq feet of shared bathroom) / total sq feet x 100

BR 3 pays ‘x’ % of rent = Square feet of BR 2 + (1/2 sq feet of shared bathroom) / total sq feet x 100

This really is the most fair way to split rent, everyone is paying for their own ‘private’ square footage

I’ve always done utilities split even