Thanks everyone for your input and help! Fiera is still very back and forth on weight, but as everyone has said, it is normal for a three year old, and yes I am very guilty of expecting more weight on a horse, my other two are heftier but definitely not obese. I just prefer to see muscle and fat rather than bones and muscle. Since then we have switched her food to Kent grain, with rice bran (Maxeglo), and a vitamin/mineral supplement. We had called a vet out to do a fecal exam to make sure there weren't parasites or worms that were undetected, and that's what the vet recommended to put her on, she said the grain we had her on was way to high in sugar and starches, and we were feeding way to much grain. Now she is on a pitcher full of grain and a quarter of a pitcher of maxeglo and vit./min. Supplement. So we will see how it goes from here!
The grain we use to feed she got two pitchers (almost a coffee can and a half per pitcher) full of the mare and foal nutrena, then another pitcher full of Platinum Performance Ultium. Her hay she has 24/7 is a mixture hay of grass/alfalfa, clover, and timothy (Really nice hay!).
Here are some new pictures of her, she's starting to look a little more like a horse rather a skinny pony, but hopefully will fill out more with age! Bottom picture is Fiera with her herd :)