A CCC transfer student is one who has completed at least 30 semester (45 quarter) UC-transferable units at one or more California community colleges, and the last college the student attended in a regular session (fall/spring or fall/winter/spring) before admission to a UC campus is a California community college.
UCD TAG information: http://engineering.ucdavis.edu/undergraduate/transfer-admission-guarantee-tag-program/
If you want to TAG for UCD, I would start attending a CA CCC now so you will have at least 2 semesters/CC units (Spring/Summer) under your belt before applying for TAG in September 2018.
Without TAG, UCD GPA admit range was 3.40-3.80 for Civil Engineering.