You have been given some excellent advice here...my concern would be to find the source of the swearing and STOP it from that end.

The first rule with swearing is to never let your child hear you use bad language.  There should never be a double standard as far as speach goes. If you do let a bad word slip don't be ashamed to own up and appoligise to your kid.  At age 3 my son shocked us when he came out with the "f word"; he had heard it from the kid next door.  We asked him what it meant and of cause he didn't know.  We had a calm chat to him about the appropriate use of words and ever after banned the use of words that he didn't know the meaning of and words that are totally inappropriate.  This also led to some good educational experiences as the kids often came and asked the meaning of words that they did not understand.  After this any swearing was dealt with as disobedience and disciplined accordingly.

When you say you have tried everything; what exactly have you tried?  We generally advise our son that it isn't a nice word and that we don't want to hear it anymore.  (He generally only says 'shit' and I have to admit, it is usually in context, like if he falls off his bike or runs into a wall).  Sometimes I hear him swear when he thinks nobody is around, so I pretend not to hear it.  If he does it in front of people, I put in him time out. 

He went through a stage of telling me to shut up over Christmas; even though he couldn't say it properly, I knew what it was.  I would instantly remove whatever toy he had in his posession overnight and put him in the corner without even telling him off.  After a week or so, the novelty of saying that seemed to have waned somewhat, as he didn't like the repercussions.

You need to be careful not to make too much fuss and for discipline of your choice to be instant and consistent.  There are many fantastic articles on discipline on the site, check them out to see if there is a style that suits you.  Don't make too much fuss when you tell her off, as she will like the extra attention.  Also, have you found out where she is learning these words from?  Perhaps you need to deal with the source of her knowledge as well as her use of it.

I tried soap in my 5 year olds mouth when he swore, but all it did was make him cry about how mean I was!  He still swore whenever he got cranky. Now I say to him that those are 'grown up' words, and only grown ups are allowed to say them, but they don't sound nice at all...  it did take a bit of reminding every day for about a week, but it seems to have worked for my son...  Now he pulls us up if we let slip with a swear word!

hi we have just started this one ourselves and it seems to be working quite well for us ...we have a couple of rules in our house which arent ever to be broken  1) you are never to say i hate u - even when angry 2) never lie 3) do not swear ..

we have more but these are our main 3 ...now if these rules are broken we do GONE FOREVER ...this is simple it entails taking away something they love --generally when u first start using this approach u start with small things like a crayon -- and it is thrown in the bin  and is gone forever never to return ...

we sat our girls down and told them this would be happening and when it did the first couple of times it ment not much as only small things were taken but when something they like was taken the effect we required had been achieved .....

our girls now now rarely say i hate u .....as this was our main problem when being told off .....and if they slip i only have to walk into their rooms and they know ......( I dont always throw away good things i do however give them to neighbours to give to their grandkids or put them in the good sammys bin )but they are def gone forever ...u can make this a game ...like please pick up ur toys okay ill count to 3 and then its gone forever ......hope this makes sense .......and helps ....

I taught my kids their first words and they were "Oh Shit".  It was hard to explain but I told them that mommy was wrong to use that and put the initial blame onto the person who taught them, which happened to be me in this case.

Set a rule about swearing, I agree no acknowledgement, sometimes they are looking for any type of attention and bad is just as good as good attention at that age.  A rule that will take away something they like.  Or better yet, let them make the rule.  Say something like I do not like swearing and as a parent I would take away a priviledge, what would you take away if you were the parent and you did not like what your child was doing?

It may be too soon for that to work but it worked for me at least in the latter years.

I think I would use a deterent but only if the initial physcological deterant doesn't work.

i know this may sound cruel but have you tried soap or even pepper when she swears? my older sister had the same problem with her son and pepper worked a real treat, he very quickly learnt not to swear!

Well little 3 1/2 olds do have their own minds so you need patience and luck! I hope this will work for you but when bad words were used...no reaction,no acknowledgement and they were not heard until a sorry was given and no bad language was used. AYours is a bit young but i always told my guys everyone can swear but it is a challenge to use your language without it. Best of luck!