Help with picky eater

I guess I’m not sure what I should be doing. My 3 year old has become very picky and mainly only wants to eat snacks. When I serve her a meal (even a very small serving), she will snack on it for hours every time or not touch it. Some food though (like milk and cereal or a sandwich that gets dried out after sitting out a couple hours) gets gross after sitting out. She will ask for a snack after an hour or so with food on her plate still. I want to tell her she has to finish her meal before she gets a snack, and I do when the food seems fine still, but when I see gross dried out or soggy food I’m not gonna make her eat that. And like I said I’ll give her a small serving, she will still either snack on it for hours somehow or not touch it. What should I be doing?