I love that she asked for bbq spare ribs! That’s brilliant 
I’m assuming this is a nursery / preschool rather than school as she’s only 3 (unless she is turning 4 in the next 3 days) - is that’s the case it probably won’t be a huge hall with a few lunch staff trying to help everyone - preschool lunches are normally much less children and it may be doable, as preschool staff have a higher ratio and will be on hand. It’s not unseen, I had this at the nursery. Even had parents not use a thermos but hand me food that needed heating. We did but that’s our policy and the preschool had a microwave and we were trained in food hygiene.
If it’s school I would heed caution. Practice at home. Cook it in the morning, put in the Thermos. Have lunch at school time and see how she handles it. If she can do it no problem, it’s not too hot and the school are happy with it, go for it. The school can only say no after the event in which case you’ll need an alternative.
Would she eat something like pizza? Seen that quite a lot in packed lunches, generally home made and served cold. Also simple store bought cheese and tomato ones but with the right container could be warm (I’m not talking leftover dominoes before I get pounced on).
Does she like the sandwich components separately? That’s another route to go if soup doesn’t work. Or pasta or cous cous.
Funniest things I’ve seen - one child always had a prawn cocktail in his lunch on a Friday - that always made me chuckle, he loved it, would shout out “ yay it’s prawn day!”. Another child quite frequently had sushi (like the ready made stuff from the lunch section of supermarkets). I remember one child opening up their box and being so delighted that his mum had thrown in some smoked salmon. One lovely Indian mum used to pack her son some delicious looking lunches - meat that had been spiced, little homemade naans.....I used to be quite jealous of it myself! Goodness knows how long that would take her. Anyway....
Would she eat a precooked chicken leg? Keep in mind they won’t refrigerate her lunch but the right container could be fine. The meat route may be an option with some dry carbs. Does she like things like triangle cheese? Homemade potato salad? Little potatoes plain? Hmmmmm, here’s me hoping any future child of mine is fine with school dinners I would run out of ideas in a week 