I think you just have to be straight, I would start off by testing the water saying that I'm considering moving in with my boyfriend, and wait for his reaction.
He will either be very positive for you....him agreeing it's a good idea after 3 years... Or he may take an oposing view be utterly selfish and say what about me.....
That reaction will tell you all you need to know
You can then decide from that how to play it.
He could be very happy for you surprize you by saying ...actually I've been thinking I should move on for a while but didn't know how to tell you...
In which case problem solved.
Until you make a definite decision everything is kind of hanging in the air at the moment.
I think for starters casually test the water.