How to get a 3 year old boy to listen?!

September of last year, my fiance and I took in his (at the time 2) year old cousin due to him losing his father and his mother is a dead beat. He was really good at first and then it was like a switch flipped in his head the day he turned 3 and is now a terror. I know it's really just the age but I am just so sick of having to tell him multiple times to stop doing something he shouldn't and having to threaten time outs for him to listen. But even then, he will just go on to doing something else he knows he shouldn't be doing. I put him on his bed for time out, have put him in the corner, have taken away toys/tv time and have only spanked his butt when it is truly necessary but NOTHING works. When he comes out of time out, he's good for a couple minutes then goes back to being hateful and doing the things he was just in time out for. I'm just at a complete loss of how to get him to listen. I have even tried to tell him he will get rewarded for being good and listening but that doesn't even work... Any advice is welcome!