This is primarily a matter for discussion and determination among the three of them: the expenses may be shared in any way that they see fit. If the can't agree and litigation over expenses ensues, there is no hard and fast rule for how a court would split expenses, though it would generally look to respective ownership interest, and also use and benefit. For example, if they each are 1/3 owners but one lives there, the court might charge the one who lives there with the expenses flowing out of his/her occupancy (e.g. water, utilities) and split other expenses equally (e.g. insurance, taxes).