What should I do? My three year old always wants to eat after dinner??!?

I have a three year old daugter. Every night she picks at her dinner and only eats some of it and says shes full. Five minutes later shes hungry but she wont eat her dinner when I re-offer it and she asks for a sandwich or yoghurt or something similar. Has anyone else been through this and what did you do? Its driving me crazy and I have to admit I am very soft as a mother.