Lots of good ideas from the above. And while ADHD has a genetic link, it can skip generations or siblings. He could be just going through the terrible threes. Get the book, "SOS Help for Parents," by Lynn Clark and give that method a try.
Hi there. Well, you mention swearing . . . that leads me to believe he is doing what he sees others do (as cuss words are not just in a child's brain. . . they have to hear them . . . ). What is the home atmosphere like? Is it at all volatile?
Kids of that age, three, have a hard time expressing emotions especially anger and frustration. I'd work with him on acceptable ways to express himself. Model good behavior, model working out a problem or working through frustration. Like play act it out. "Oh, those car keys are lost again. This makes me so mad. What can I do when I'm mad? " Say it all out loud and in an exagerated way. Then talk about what you can do when you are mad. Take deep breaths, ask someone to help you, use your words to express how you feel, count to 10, go somewhere for a minute to cool off, etc. Then it is easier to remind him to do the same. Talk about what HE can do when he is upset.
And then make it clear what he can't do. Set a very clear boundary and have consequences when he doesn't adhere. Time outs, love objects taken away briefly, a drop of vinegar on his tongue for foul language all should work. You remain calm however. And make sure the others he is around do so as well and start being good role models too.
Add/adhd is a complicated diagnosis that requires a lot of data. You'll know more down the road when he is in school. Try these other things to help the situation in the mean time. good luck
He is pretty young for such a diagnosis. Your might want to try some diet changes (cutting out sugars, sodas, juices, etc.) and increasing fruit and home-cooked vegetable. You may also want to seek advice on behavioral techniques.