I breed GCCs and extensively research their diet, behavior etc.
At 3 months old, she should weigh more, I would say from 58gr to 65gr is the norm. She is not eating enough in my opinion.
Pellets and seeds should not be the main diet, but they need both (sorry some people are against seeds but they are seed eaters, I didn't make it up, it is what it is). Denying them from seeds is denying them their natural feed. Not all seeds are created equal and some are necessary for proper healthy health and mental development (good quality seed mixes are sold my Higgins). Pellets are made with seeds and pulses, they get more variety with original seeds than the grounded ones in the pellets. They add some vitamins, folic acid, biotin. They can also get all this with a variety of fresh food made into a mash (veggies, pulses, grain, sprouts etc).
So if you can't feed a variety of fresh food daily (or some people can't be bothered), the pellets are a good choice to have in the cage. I keep both seeds and pellets available. What if you are going away and the bird sitter can't be bothered?? You want your bird to be used to eating both.
At 3 months old, it would help to moisten the pellets as she is newly weaned (8-10 weeks old is the approx. weaning age).
Where did you get her from? I would stick with the diet she is used to at first for a smooth transition. You can always ADD new food along with it.