I personally think that the example you gave of "I was talking to just mommy" is not at all nasty, even with the "sassy" intonation, because people do communicate in this way every day. He is trying to tell others that he is "having an A and B conversation, so C your way out of it" I see nothing wrong with it. People need to be put in their place when they interupt a conversation between two people, you son is imporant, and he should be recognized as such when you have having a one on one talk with him he shouldn't have to deal with others butting in just because he's 3.
As far as the "bad words" I don't blame you for not wanting him to say those things, but I will just shortly agree with most others that he has to hear it somewhere to pick up those words, so monitor what you and the people you bring around you child say. Also whatever is on TV has a huge effect, and this is why we no longer have cable since before my daughter was born, so that she isn't watching anything we don't want her to, only what we get on DVD for her. Just a thought too, lots of commercials on TV for kids shows can be pretty violent, so I wouldn't be surprised if there is an element of sass there too, but I couldn't be totally sure on that again since we havn't had regular TV in the house for a long time.