Lisping is normal at three, so I wouldn't treat that in a child her age. She should be understandable about 75% of the time when she speaks, but all her sounds aren't expected to be in place at three. Lots of them are still developing.
If you take her to your local school district's early childhood special education department (look for something called Child Find) they will screen her for free and tell you if she needs speech. If she does need it, it's free through the schools but kids have to be VERY far behind to get therapy for speech sound disorders through the school system.
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