Hi Poutine,
By all means have your daughter checked out by a doctor to see if she has any problems. However, certain people (my husband is one of them) simply have naturally a very low "calorie burn" rate. Folks like this are probably highly adapted to survive in adverse conditions, but we don't have those conditions nowadays.
My husband was always "the fat kid" in school, and he has always had a hard time keeping the weight off. I've worked hard to change his dietary habits, and here are some things that have worked for us:
1. High fiber foods fill you up without adding too many calories. Grapes work better than raisins, raw carrots (those little baby ones are nice) work better than cooked carrots, and whole-grain brown rice works better than pasta if you want to feel full without taking a second helping. Use fresh food whenever possible--frozen food second--canned food last.
2. Cheese, meat, and nuts are all calorie-rich protein foods, and should not be used in the same meal. It's cheese OR meat OR nuts, not cheese AND meat AND nuts! In measured amounts, please. (I'm not saying you shouldn't have some of each, but it is easier to limit input if you have just one in a meal.)
3. Empty calories--in candies, pastries, and the like--are your worst enemies. Because there is very little nutritional value in standard white flour and white sugar, your body wants more food, to fill in the missing nutrients. And if you give it more empty calories, it just wants more... You never find the "set point" for your appetite. Around here, we use frozen fruit instead of ice cream in the summertime, and I never buy treats to keep at home. If we want cookies, we have to either bake them from scratch (small batches only) or go out somewhere to have them! (I keep the sugar in a ziplock bag in the refrigerator, because I rarely use it.) This stops the mindless "snacking" on sweets.
4. If you are a slow burner of calories, you need to have some sort of organized exercises/sports in your life. In the doings of everyday life, you are a natural conserver of energy, so you need to have something that goes beyond everyday life. I'm not suggesting competitive sports, but something like dance classes for little ones (with pretty outfits!) that will get her moving more, and practice between classes.
5. Please don't use food as a reward. For years my husband would go on diets, and tell himself, "When I lose 10 pounds, I'm going to get myself the biggest chocolate milkshake in the world!!" And then he'd wonder why the pounds wouldn't stay off! Hugs and kisses and warm praise ought to be the best rewards for a three-year old.
6. Finally, on fruit juice--I use fresh or frozen juice diluted with 1/4 water in my house. You can't tell the difference taste-wise, but it removes a few calories. And cool, filtered water is the standard drink between meals.
Good luck. And give your little one a hug from me.