I never heard medical schools reserve seats for their UG. Whether it is Harvard or xyz school.
Many schools have EAP (Early Assurance Program). Once you matriculate to an university, you go thru that EAP process and if meet all requirements, you may matriculate to a medical college.
Yes, UCR has an EAP. Neither UCI or UCSD has EAP based on my old knowledge. Have not checked their site recently.