You should ask your Pediatrician.
babies have various appetites.. but yes, should be fed on demand or at least every 3 hours, unless she is going through a growth spurt (3 months is a growth spurt time) and during spurts they drink more and more frequently. So, don't go according to just the clock... she also needs to be fed when she indicates hunger. And your care giver needs to be aware of that. Feeding on demand is still the pattern. At least for me, this is what I did.
At 3 months old, my son was drinking about 4-5 ounces every 3 hours or less. But that is him. I don't know that ALL babies drink this amount.
If your baby is drinking 3-4 ounces and completely empties the bottle... then I would try adding an ounce or 2 more.... and see if she will continue to keep drinking this amount at her feedings. The reason is... if your baby is emptying the bottle each feeding with just 3-4 ounces this "may" just be making her satisfied...but is more like a "snack" and... she may be ready for more, even if she seems "satisfied" with her current amount. Just try it and see. Give her more, and see if she can indeed drink it at this increased level.
Every baby will vary... but there are "standard" guidelines as to how much a baby this age should be drinking. SO ask your Pediatrician.
No matter what, I fed my babies on demand throughout no matter what age. I did not just go by the clock. Sometimes my babies needed to drink even every 2 hours or less. Or cluster fed. But the usual was every 3 hours. I also let them "tell" me when they needed to nurse/be fed and went according to their hunger signals.
I would supply your caregiver with an extra bottle or 2 just in case... always provide a little more for those days she may need more. For the 1st year of life, breastmilk/formula is the PRIMARY source of nutrition for a baby after all.