Y'all please help me!

My 3 year old son will NOT STAND IN TIMEOUT!!!!! People keep saying "persistence" and "every time he gets out, put him back". But he won't stand for even a second. He throws himself out immediately. I have to physically stand him in the corner. Even then, he lifts his feet so I'm basically holding him up. He will NOT. I'm at my wit's end. I don't believe in spanking but a few seconds ago I got so frustrated I popped him twice on his leg and he didn't care. He screamed in my face and threw himself out. Guys I'm at my breaking point. If I ask him to SIT in timeout, he lays down. Its literally a power struggle. But if I let him win, he basically doesn't have to sit in timeout. If he does and he's compliant, he whistles and hums like he doesn't care. Then when he gets out, he very obviously "tells me what I want to hear". He's becoming such a brat. Telling me no, hitting me, etc. And it's so hard to discipline him because timeout is literally a whole wrestling match and it's not productive. Any suggestions?? Any other forms of discipline? I don't believe in spanking and the two times I've broken and done it, it makes him LIVIDDDD. He doesn't stop what he was doing, he gets worse. He's such a strong willed child. Please help me!