Hi C.,
This may sound silly, but do you really want him out of your bed? If you are both happy with the arrangement, maybe it is ok right now. (I don't know if you are married, but assuming so, of course your husband would also have to be ok with the arrangement) I pesonally don't think 3 is too old to still be sleeping with you.
In today's day and age when so many children are in childcare all day. Being physically close at night is a way to reconnect. Of course, if you, or your son are not sleeping well because of the arrangement, that sort of defeats the purpose.
If you really want him in his own bed, let him know in no uncertain terms what the expectations are. Expect some fight and be persistent. If you are committed to getting him into his own bed, eventually the new pattern will be established.
Try to have a consistent routine, bath, story, tuck in. Reassure him and listen to his fears (check out the closets and under the bed if necessary) maybe provide a night light. My 9 year old still falls asleep with a sound machine. (she likes the faint city noises selection) (funny, because we have never lived in the city!) We bought the machine at Brookstone and it has numerous sound choices as well as a sleep timer.
Hang in there. This too shall pass.
J. L.