A: Manners are pretty advanced skills, and parents often get disappointed when they have unrealistic expectations of their young children. A three-year-old is still getting the hang of his body, and how to control it -- and quite honestly, he couldn't care less about manners. That's just a developmental reality.
There's not a lot you can do to force the issue. Your best bet is to give him a lot of praise and positive feedback when his manners improve even a little -- and ignore the poor behavior. You run the risk of reinforcing bad manners by giving them too much attention. For now, focus on staying at the table for mealtime, using an "inside voice" when the family eats together, and keeping food on his plate. Better manners are more realistic over time, but certainly at age 5, 6, and older.