D.W.
Use different methods until you find the one that's most effective for your child. If it were as easy as time-outs for everyone, there wouldn't be volumes of parenting books with slightly different approaches.
Our 2 kids (ages 2 and 4) have completely different personalities. What works for one doesn't work for the other, so we change the approach to discipline while being consistent for the things that require discipline.
If he were my child, I would grab his wrists, look him right in the eye and firmly say, "We do not hit! Do you understand?" A few minutes later, I'd ask, "Do we hit? NO!". Three is a tough age, you're going to have to repeat ad nauseum until it sinks in.
Start taking things away such as favorite activities, favorite toys, treats, etc. He's old enough to understand cause and effect.
And, start making a concerted effort to praise him for all the things he does do well hoping the praise is more frequent than the reprimand. Our 4 year old is finally understanding that he prefers "nice" Mommy and Daddy vs. "mean" Mommy and Daddy.
Good luck!