T.B.
I would definitely make sure that his room is safe if he gets up/ out. I think at 3 he is probably ready for a toddler bed/ twin bed with rails. You might have to spend the first few days outside his room, taking him back to his bed and putting him back in it- over and over. I would not trust him given his history, but being there and on top of it, and able to take him back to his bed should eventually help him get the picture. I would talk about how he's a big boy, and big boys stay in bed. Leave him with some books or cars in his bed to keep him occupied until he falls asleep. Also, if he is getting any kind of help for his autism, I would talk to your doctor/ OT/ PT and see what they suggest given where he is. Sounds like he is very smart, so you just need to be a bit smarter and stay one step ahead of him. ;)
Good luck,
T.