Looking for tips to treat SM please...

Hello, we just found out that our daughter has SM. She talks non stop at home and is very social with us (parents and siblings) but as soon as she's outside of the home (School, playdate, visiting the cousins), she completely stops talking. Same if some people come to the house, she will barely talk. Do you have any recommendation of how we could help her, what should we do and what we should not do so that she slowly starts talking?? She is 3 years old by the way
Please let us know, thank you.
Hagi