While it is probably better to wait if you can, if she can hold her head up on her own & possibly even lift her head away from the back of the carseat, and seems interested in putting things in her mouth she might be ready for it.
I wouldn't give her anything but rice cereal mixed with formula or breastmilk - very thinly. My cousin's son had severe reflux and was on rice cereal in his formula from his second week.
It really is better to wait though because babies' digestive systems are not that mature at 3 months, and formula and milk do give them all the nutrients and calories they need at this age.
Also some doctors think that giving babies solid foods before 4 months (or some even 6) may increase the risk of allergies developing.
You may want to check with your doctor to get their take on your situation. If you feel your baby is not getting the calories they need you could consider supplementing with formula - or perhaps increasing times between feedings.
I've always waited until at least 4 months, but usually six to introduce solids. My second just couldn't wait that long. He was huuuuungry!!