my grandaughter is 3 and she saids bad words all the time.

first whoever she is learningthem from needs to stop talking that way around her, and if shes getting from tv then there need to be rules as in ADULT language shows when she is NOT in the room and cant hear them

we used lemon juice, one drop on finger and onto her tongue, if she fought we just rubbed on teeth, it still gets there. didnt take long. Wont hurt her if its not an all the time thing and just a drop

When my daughter, at about that age, heard a bad word and started repeating it, I told her that it wasn't a nice word and she couldn't say it around other people. I told her she could go in her bedroom, close the door, and say the word as much as she wanted, but in her bedroom was the only place the word was allowed. Over the next few days, I heard her go to her room and say the word a bunch of times, but she soon got bored because she received no attention, positive or negative, for her behavior. No one was around to hear, no one responded to the word at all, and after less than a week, she gave up on it altogether and found more interesting things to do.

She learning it from someone.

Who is she hearing these words from?

If you think you know, you might want to have a chat with them.

Find out where she's hearing it and point out that it's not good when (insert name) talks that way. That is not how you should talk.
In our case the culprit was a Disney character, Lotso, from Toy Story. He called someone an idiot. (nice, Disney) So when she repeated it, we said that Lotso talk is not nice. She got the picture and now says uh oh Lotso talk! When DH slips sometimes and we've never had a problem with her language since and it's been about a year.

Of course,= it is a good suggestion to try to stop her from hearing this kind of language but the deed is done. And kids of this age will often latch onto a behavior *because it ticks you off.* So it's important to calmly say, "That's not a nice word. Please don't use that language in my house" (or in this restaurant or whatever.) But it's really important not to get too worked up about it. It's only a word. My DS has started to say "damn it" this week because he heard it in a movie (my mistake!) I have found that it is very helpful to offer a replacement phrase, like in this case he seemed happy to switch to "darn it" or even "oh, man!" I've had to remind him a couple of times but I think he's figuring it out. Good luck!