as part of a couple who also lives with one other housemate, i'd recommend against it!

the problem is we are basically living as a unit, starting a "family" (us and our cat, anyway), and that automatically makes our housemate the outsider, the odd one out. everything is viewed by everyone as being hers or ours, not as three individual people. so there's "P and C's dishes vs. K's dishes," "P and C's laundry vs. K's laundry," "P and C's guests vs. K's guests," etc.
the fact is we realized really quickly we don't like living with someone else in the house, because she is intruding on "our" space. i know that's horrible, but it's inevitable because within the couple, we don't mind each other's messes or get pissy about housework or anything, because we see ourselves as one unit. but we can't help but get pissy about our housemate's dishes, etc. and she gets pissy about our dishes in a me vs. them manner, too. the result is our housemate never does ANYTHING around the house because she views any given chore as being 2/3 OUR responsibility and only 1/3 her responsiblity. so she has literally never taken the trash out, never cleaned the bathroom or kitchen, never washed the communal dishes, and leave us to do all of those things. and because she insists on washing her dishes seperate from ours, she won't wash them properly and dishes always have food stuck to them, or she leave them lying around and we have to wash everything... it's just a bad scene all around, with a lot of ill will on both fronts. i haven't even spoken to her in two months, no joke. luckily we only have 3 more months on our lease.