It's hard as heck to get a 3-year-old to be nice if he's prone to walking on the nasty side with the family pet. Kids this age still don't quite know how to express themselves, so if they're frustrated, they may lash out at anyone who will take it, especially a helpless dog who can't talk back. The best you can do is ramp up your expression of displeasure with his actions and teach him how to be nice to Fido. The next time he reaches out to touch the dog inappropriately, take his hand and say, firmly, "We do not hit." Then show him how to treat the dog right -- use his hand to pet the dog gently, and, with a sweet voice, say, "Nice doggy." Do it enough and eventually your child will do as his mom does. If he's violent to other people or keeps up the aggressive behavior, though, it's best to see a doctor.