Beware of getting what you wish for. If he's had two marriage failures already, why would he risk a third? He's already getting everything he needs out of his relationship with you - sex, companionship, friendship and his own personal space in his own home without risking losing his savings and paying maintenance if it all goes wrong again.
It sounds harsh but I think you're on a hiding to nothing. If you've been going together for three years and that hasn't convinced him that you are the person he wants to spend the rest of his life with in a firm commitment, I'd get out now if it's marriage you're seeking because it sounds as if he has no incentive or desire to tie the knot.