Your three year old is using bad language because it gets a reaction, so make sure your reaction is minimal, and calm. Explain that there are certain words that make mummy and daddy sad, and he must not use them. If he persist, you must confirm that the words are not to be used, and you will be corss with him, then instigate an escalating form of punishment - removal of a favourite toy for an hour, missing an hour's TV, or a favourite programme. If he responds by dropping the language, assure him how happy you are, and how nice it is that he speaks properly, and that it is his way of loving you, and your way of loving him. By re-enforccement of the message, combined with punishment, he's get the idea. You must be fair at al times, and strict with your regime, and back each other up as parents, or he will play one of you off against the other. I'm taking it as read that none of swear in front of him.