Every once a while or three-year-old comes to sleep with us. We've had trouble with him sleeping in his own bed since he was about two. One thing we did was to come to a toy store and let them pick out his own stuffed animal so that he knew it when he had a stuffed animal that it was time for bed in his bed.
We also have him in a full-size bed and we got a tent for the bed one of those sleeping tents? It worked great. We made a big deal out of it that it was just for him and no one else could fit in the little door to get into his bed. It was his fort.

He still comes to sleep with us every now and then. He can climb up onto the bed himself. When he starts kicking and one of us gets up we usually end up sleeping in his bed. It's a full-size bed a twin, but it's comfortable. At least one of us is getting a good nights sleep.

I did like the suggestion of putting his mattress next year mattress so that he knows that the security is there if he wakes up and might help them to stay in his own bed.
Also are you in a routine for when you put him to bed? Something that's very step-by-step. Brush your teeth, read a story, lay down in bed, etc. etc. kids at that age are very into routine and structure.
Personally I suggest getting him into a real bed, not a toddler bed, and actually having a place for you to get a good nights sleep. Parents need a good nights sleep!

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