My 3 years old is a bit on the shy side, but nothing alarming. She manages to play with friends and interact with adults.
She has been at the nursery school since she was 1 year old, and we only heard good feedback from the care givers.
So, all good, except when there are group activities, i.e. activities for kids led by an adult. For example, children in circle doing some simple dance demonstrated by the adult. Every time she has been in such a situation (at school, or other places), while all the other kids her age were participating (with more or less involvement, as 3 yo do), our daughter refused strongly to participate. She would sit and look at what is going on, and this will be it.
The activities done are of the kind she would happily engage to at home, when just with us.
There does not seem to be any stress, there is no fuss. Just, if we suggest (kindly) to join the fun, she would say 'no'.
I would be happy to hear any advice on how to help her going forward with this.