You can can't consider yourself having been in a three year relationship if he keeps ending it every other month so divide the actual time you've spent with him in half (or less).
I recommend you read the book in the link below. You are accepting back a guy that only wants you for 'new relationship energy' sex. He dumps you when it gets routine only to come back when he knows you're pining for him and will **** him like you just met him.
The book:
http://www.amazon.com/Facing-Love-Addiction-Giving-Yourself/dp/0062506048
Stop being a love addict and remaining with a man that won't love you back. This merry-go-round he has you on is making you emotionally ill and it's whittleing away at your self worth one return at a time. Get off the ride.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion