How common is it that community college transfers spend three years at the UC rather than two?
Extremely common.
See page 2, http://www.ucop.edu/institutional-research-academic-planning/files/Transfer_Student_Brief%2011_20_15.pdf
Over half of UC transfers graduate in 2 years.
Probably varies by how well the student’s prior school covered the lower division prerequisites for the student’s major at the UC. http://www.assist.org can give you an idea of how well various CCs cover your major’s prerequisites at various UCs and CSUs.