If you have no other choice but to live with your MIL, then you would do well to investigate a wonderful communication technique called Non-Violent Communication. It will help you to connect more effectively to your MIL. If she believes you care about her feelings and emotional needs, she will be far more likely to be able to hear and respect your needs.
I hope you will understand that your MIL is also your daughter's grandmother, and as a granny of 4.5 years myself, I have made the amazing discovery that I love my grandson as much as I loved my own daughter. I'm grateful that my daughter and son-in-law have parenting styles compatible with mine, because I want the best, happiest, and healthiest possible life for him, and will do anything reasonable to help him achieve that. We have a very special and tender bond, which is normal and entirely appropriate, and I add a great deal of value to my grandboy's life. If I lived with him, it would be very hard for me to see his dad, my son-in-law, handle him in ways that I might find too strict or unfeeling. Fortunately, I think he's a terrific dad, so we don't have those issues. But I do see how that could be a huge problem for me if it existed.
I hope you'll consider, as best you can, that your MIL may feel similarly about her grandchild. I'm not saying you are necessarily too strict, because I have only your side of this one brief story to go on. But I have applicable questions that your request leaves unanswered: Do you have an authoritarian parenting style? Does your MIL have a different style? Does this sort of issue arise often? What is your child's side of it; was she indeed hungry? (My grandson doesn't always realize he's still hungry until 10-20 minutes after he leaves the table.)
I guess I'm wondering, being a person with an empathetic parenting style who was raised by a very strict and unbending mother, and whose live-at-home granny was my only source of tenderness, whether your MIL may have issues similar to mine. I feel genuine anguish for children who are raised "by the rules" with little concern for their emotional needs. Again, I don't know that this is your style; I am only guessing based on several statements in your request.
I'm also wondering where your husband is on the scale of "authoritarian–authoritative" parenting. Does he have any opinions on how his son should be raised, and is he willing/able to talk to his mother about your disagreements?
If my 3 yo grandson were left upset by an abrupt or even angry mom in the kitchen as she hurried off to get ready for church, if he had been presented with an all-or-nothing scenario (eat now or be hungry till after church, which is an incomprehensible stretch of time for a 3yo), if I wanted to help soothe his feelings/needs as I perceived them, being worried that his mom wouldn't, if I were afraid that his mom had little understanding of what "rules" might be reasonable and age-appropriate for a 3yo child, and if I were afraid to state my concerns because the child's mom had no room to listen to them, then I can't help but wonder if I wouldn't, at least occasionally, try to meet the child's needs (as I perceived them) without asking permission from the mom, or even in defiance of the mom.
I understand that you feel undermined in your child-rearing efforts by your MIL. It is entirely possible that she does this deliberately or thoughtlessly, and tries to "buy" your daughter's love by countermanding your reasonable decisions. Again, if she must live with you, then this is something the two of you would do well to talk through. But she may have a point that would enhance the raising of your daughter, and if your child becomes a helpless pawn in a power struggle between two important nurturers, no good can come of that for her.
So, with that possibility in mind, I think the question is of whether or not you are being condescending is possibly more important than you have considered. I hope you will listen compassionately to your MIL's point of view – she did raise the man you married. Consider her contributions to the family from the point of view of your child, who may need her sympathetic advocacy (or not – I'm not there to see the whole situation). For the sake of family peace, I hope the two of you can find a way to some agreement.