Hi R.,
Hmm. Wow--that's a lot of time you are spending in the kitchen at your dinner table. I feel for you!
My niece and nephew are slow/picky eaters, and I know that if they are denied snacks, that they will eat their meals at a normal pace and consume a quantity that satisfies their parents. My niece is a slower eater when the t.v. is on, so my sister removes all distractions from her if she slows down.
My son is a good eater, but he will not eat well at mealtime if he has had too many snacks or too large of a snack--so often, a snack is something small (like a cheese stick) and snacks are not automatically offered every day. I just try to keep mealtimes at around the same time every day and I provide him with a calcium, protein, complex carb, and fruit/vegetable option at each meal so that I know he is satisfied and will remain relatively so until the next meal.
If I were you, I don't know that I would want to spoon-feed a three year-old either. I understand your concern about this, especially since you have a baby. I think part of growing up in a family is learning about mealtimes (in whichever form a family defines that for themselves); so, I don't think it's a unreasonable request for you to ask your daughter to eat a meal within a certain time period. An hour and a half seems like an awfully long period of time to devote to eating (unless you are eating a five-course meal in a fancy restaurant)!
Like you, I would probably remove the food after a reasonable amount of time (45 minutes? an hour?)--however, since you mention that your daughter has had trouble maintaining a healthy weight, I would ask your pediatrician about how to transition her to eat more like a big girl. Perhaps you can entice her with some Pediasure if calories are a big concern. Some kids just don't need to consume very many calories--girls certainly less than boys--so maybe this is an issue of learning what a socially acceptable mealtime is versus a real concern over the calories she's getting (since you say that she's achieving the appropriate milestones).
This might just be something that she outgrows on her own if you just stop spoon-feeding her and/or remove her plate after an established time period. She will have to adjust somewhat once she begins school since there is only a certain amount of time allotted for snacks and lunch. She will probably do this just fine as a result of "peer pressure."
On a side note--my son announced today that he didn't want a sandwich at lunchtime. He only wanted fruit. I said okay, but that lunchtime was now and that he should not expect to get a sandwich later since lunch would be over when he got up from the table. He said okay. Sure enough, an hour later he wanted a sandwich. I repeated that lunch was over, but that he could have a very small snack now (three crackers), but that would be it until dinner. I also told him that when it's lunchtime, we eat, and if tomorrow he didn't eat a full lunch when he was hungry, that there would be nothing more to eat until dinnertime. He's never done this before, so it's a bit odd (which is why I gave him the very small snack)--but hey, I don't want to be making lunch all day long.
In general, I view snacks as options only if he's eaten well at the previous meal and it's still a while before the next meal. This works for my kid and my family's beliefs; I know there are a lot of opinions on eating out there...
Happy Eating!
:-) D.