A student who wants to finish in three years needs to think a year ahead in terms of summer jobs, undergraduate research, etc. as well as course work. For example, a summer job between the student’s second and third year should be the kind that students in his/her major normally do between junior and senior years (not sophomore and junior years).
Note that there are alternatives for a student who is substantially ahead in course work but does not want to stay in school four academic years. An example would be 7 semesters with a co-op, or 6 semesters with two co-ops. The student graduates with substantially more work experience than others in his/her cohort who graduate with him/her.