I don't really compare weights to age as each snake is different and each grows at different paces. I would just keep an eye on her and keep measuring her weight every month or so. Only worry when she starts looking overly obese (when she's older) and/or when she starts loosing significant weight and starts looking triangle-y.
It does vary, I've got a little female that's almost a year old and about the same weight as your 3 month old. She's a picky eater but lately has been chowing down on rat pinks and has a better feeding response compared to her other half who's sitting at 630g and not wanting to eat his rats.