She's only 3 and is still growing a lot, so it's not unnatural for her to be a bit 'under'. You have to be careful with babies because it's actually worse for them to be over weight than it is to show a bit of rib. I would talk to your vet about her weight if you have concerns, and possibly work out a diet for her, but keep in mind that she's very young and she's not nearly done filling out. If it's just a bit of rib, and she checks out with a vet, I wouldn't worry too much. She'll fill out in time, she just needs to stop growing so that food can finally go to her waist, haha.