My 3 year old girl is attending pre-school 5 days a week from 9 - 12.30a.m. She has always been a pretty normal kid i.e. no major temper tantrums etc beyond the expected. But since starting in September the teacher has now had call to speak to me twice regarding her behaviour around obeying orders(eg sitting on ground when being asked to stand) and wanting toys she has put down the minute another child begins to play with them(which has sometimes resulted in pushing etc). This problem only seems to arise or become a problem when in the class enviroment and so is causing me some worry.

I should state that she is an only child but has lots of cousins close by with whom she plays regularly. There has never been an issue regarding sharing etc outside of the school (and this still is the case).

Any advice would be greatly appreciated

I would ask the pre-school what strategies they are using to help your daughter. If this is happening in pre-school only, it is really a problem for them to solve and not your responsibility. I am sure that the situation with so many children there maybe not helping your daughter who is used to being an only child.... however, she will not be the only child to behave this way. She needs the pre-school supervisors to manage her behaviour as you aren't there at the time... and she doesn't behave this way with you.
Sure, ask them how you can support them , but they need to initiate some strategies to enable your daughter to learn to cope better in the pre-school environment.
Good luck....