typical preschool program does not offer school bus service. Sometimes there is preschool program set-up in the daycare service you choose, you just pay extra $, you send in and pick up kid at daycare regular service time.
as long as there is proper car seat, and the school bus service is available for preschool program, 3 years old kid can take the school bus. Usually it would be small school bus.
It seems there is something wrong, I can not send you message, even I can see your reply. U can reach me at the number I left before 7:00PM when u r free.