How do I get him to stop sleeping on the floor??

Dont fight it.  Let him decide when to move to a bed.  Some people do better with really hard surfaces to sleep on whereas others like really soft and fluffy mattresses.  Perhaps find him a sleeping bag that he can put where he wants to sleep.

To me this is not worth the frustration of trying to control an aspect of his life that he clearly told you how he feels.

i would just let him.  it wont hurt him and if/when he is ready he will move back onto the bed.

Its for comfort. Mine is 4 and sleeps on the floor under the coffee table. He'll grow out of it.

My boys are 13 and 11 and they still sleep on the floor in my room. Up until 6 months ago they slept on their bedroom floor. I took away their bedframes,they were breaking. Btw how do boys break metal bunkbeds?? So since then their mattresses are on the floor and they sleep on that. They have been sleeping on the floor on and off since they were each about 3.

If he prefers it, why does it stress you so much? Kids have quirks that they grow our of and into. Has he told you why, have you asked?
I, personally would just let him. But him a kinder mat for the floor and just leave it alone. This is extremely minor. This is something I would totally let go, it's really not worth the fight at bedtime as you get him all worked up and make it harder for him to settle into his "zen" for sleep. He likes the floor, great! get him a pad for the floor and just wait it out.

My middle son did this until he was 9.  I tried to get him to sleep in his bed at first, but decided I needed sleep to.  And so I let it be.  He eventually slept in his bed (by the way, when he slept at friends, he would sleep on their floor to!).  He is now 19 and perfectly healthly.  He's even a runner.  Has done very well in 5k's and mini marathons, so I know it hasn't affected is health.  He is in college with a GPA of 3.96 so didn't hurt in that department either.  I just feel it will pass.  Sooner or later, but it will pass.

 This is a complete non-issue.  He's fine.  At age 3 he's completely aware of whether or not he's comfortable.  You are more likely to hurt YOURSELF putting him back into bed multiple times each night.

My son did the same thing when he was a little over 2, let it go and in time he'll switch back to bed. I asked the doctor about it, and he said that children's minds at that age work so fast that it is impossible to figure out.

Thanks everyone I feel so much better now that youve all told me its not really a problem. I just felt like a crappy mom cause my son wont sleep in a bed.

Honestly, I wouldn't worry about it. If he's comfortable sleeping that way, let him. Like others have said, it's most likely a phase and not worth a big battle. If anything, sleeping on a hard, flat surface is BETTER for your back than sleeping on an overly squooshy mattress. Has his pedi told you that he shouldn't sleep on the floor?