At three months, she should be starting to ruminate. Try feeding her the grain before the bottle when she's hungry but not foccussed on milk. Add in a handfull of alfalfa or something alse with some stem to it. How much milk is she getting? At three months, she should be getting the milk as a supplement and not as the main source of feed. Cut her back slowly as soon as she accepts the grain and hay into her diet.

We always offered grain, hay and fresh water to our replacement heifers at two weeks of age. To get the calves used to it we would offer it by hand after giving the bottle but I'd also try to let them suck my fingers while they were down in the feed bucket to get them used to looking there for the grain. Depending upon how much milk replacer you're giving her she may not be hungry enough to have an interest in the feed right after the bottle and then when she is hungry enough she doesn't know enough about the grain to eat it. Cut her milk back by a fourth each week and offer her the feed. She'll get the hang of things.

Janet

We always had fresh calf-starter ration in front of the bottle calves from the very start - and after each feeding, or any time we happened to be passing by, we'd cram a handful of feed into their mouths.
By the time they're 4-6 weeks old, they should be eating at least a pound and a half of high-quality calf-starter ration per day, allowing you to stop feeding the bottle altogether, and gradually, but rather quickly, increase their grain feeding to 5# per day.
3 months is way too old to still be feeding that expensive milk replacer - I was shocked when I had to go purchase a bag this spring to feed a calf whose first-calf heifer mother refused him - $80!!! I doubt that I'll recoup the expense of the milk replacer and calf-starter when I sell him, 'cuz beef bottle calves just never seem to grow off very well compared to Holsteins. He's the typical little stunted, pot-bellied beef bottle calf.