How do i get my 3 year old to follow rules?

Me and my husband have had to resort to some new rules for our daughter because she wasn't eating enough and wont listen, one of our new rules(that we had when she was younger) is that she has to sit at the table and eat and if she leaves without asking no candy, juice, or desert, she today did very well with dinner but didn't ask to leave the table after we had just said if she didn't finish her dinner no candy or pie after dinner, she then walked up to me in the kitchen and said in a very cute voice "candy please" i told her she had to eat dinner she said "no im full" i then had to say if you so full how cant u fit candy in your tummy she said she left room, i guess our main problem is she will eat half of dinner claim shes full eat dessert and then right before bed cry because shes hungry again( she eats between 630 and 7pm bed time is 830-9 depending on the story and bath time, how can we get her to listen to rules that no one else but us follow in the house( my mother in law and father in law live here also and this is their first and only granddaughter)