You can buy multiple quantities of the same product to start with and then ask for one from her. Also buy few things for yourself which she might like but give them to her only when she shares some of her stuffs. This works! Kids at this tender age tend to imitate what parents do :). So try to analyse yourself from her perspective. She might have learnt it indirectly from you based on how you protect your mobile and laptops ;)
Next time bring a chocolate and don't share it with her (just pretend) when she asks for a share. Tell her that it really feels bad when people don't share. Tell her about good and bad behavior. Tell her that sharing brings happiness to everyone's lives. Tell her stories about sharing.
you will have to show her interesting things and then tell her that you also will not share with her because she does not share with her. my 3 year old wuold do the same and she finally started sharing after i kept doing this repeatedly