Hi is there a true way to put a time frame on when a skull fracture and healed rib fractures could have happened on an infant 3 months old?

Yes. Yes, yes there is. Be sure and tell the truth to all medical providers. .
Sometimes. There are pediatric radiologists that can often offer an opinion on the age of any fracture based on any signs of healing on the x-rays. No healing at all would suggest more recent injury. It is not an exact science but does provide some information.