If the rent covers the mortgage repayments why do you want to get a third person on the mortgage?
Your brother won't be able to get a mortgage on his own for a share of the property as the security (property) isn't and won't be in his sole name. If there will be three people on the title deeds, then all three will have to be on the mortgage.
Does he have a lump sum to buy into the property? If he does then as above you could take a mortgage in three names for the remaining €200K, he takes over half the mortgage repayment, pays the agreed buy in amount and the deeds are transferred into joint ownership. Depending on his lump sum you could alternatively do this for 1/3 and 2/3 ownership etc.
The lender will have to give their authorisation and will hold all mortgage holders equally liable. Its also important to have an agreement in place for when one of you wants to sell the property.