All 3 of you would need to go on the loan, because you cannot receive a gift for the down payment on an investment property. So as far as the interest rate goes, it would be based off the LOWEST of the credit scores between the 3 of you. Between you 3, you might want to draft up some sort of ownership agreement to specify who is entitle to what. Talk to a CPA or attorney for this. I believe this agreement could also dictate who is responsible for what and how all taxes would be split.
But even though everyone trusts one another you should still have a lawyer draw up an agreement. Better safe then sorry, because a lot of friendships and marriages are tested when there are large amounts of money involved.