Yes - 3 year old's can be a challenge! I think first we must reaffirm ourselves as parents that we are doing our best and trying to meet our child's needs. For my 3 year old, he enjoys spending time rough housing each day with me. It helps him get his energy out and be aggressive - I think of it as how lion cubs play and learn...so I try to make time each day for this since my wife isn't such a big fan of "fighting" with him.
Other suggestions would be spending time one-on-one with him, getting him involved in some sort of sport or activity where he can use some of the aggression, and of course setting limits. In cases where my son starts lashing out, I try "playing" back. He tries to hit and I counter...or pretend to be horribly injured and really do an over the top reaction. Sometimes it turns into peals of laughter - at least the best of the times. Kids seem to really enjoy role-reversal, where they are the strong ones or are in charge and we are weak. Although this is not an option all the time, a few times in a day can really improve your connection with them.
Of course professional help is always a good option - hope you are able to get some help. Good luck and strength to all of you fellow 3 year old Mamas and Papas.