Desperate for some advice with my 3-year old?

I am really at a low point right now. I feel like a terrible mother as I no longer have any control over my son. I've used the time-out method for about a year and it's no longer having the desired effect. He just sits there and as soon as his 3 minutes are up, he's back into trouble instantly. He destroys everything in his path, suddenly a horrible bully to his 1-year old sister, and laughs at any stern word and time out he's given. I really don't know what to do any more, I don't give him treats if he's "", and he's praised hugely when he's good, and he always has my attention. I'm finding myself on the brink of tears by the time he's gone to bed. Any advice would be much apreciated. Thank you