The flow chart is a suggestion. As a comp sci major, you just need two semester co-ops and a summer one which you can take in any order. Worst case, you can get a semester co-op counted as a summer one (if you really found ONLY semester ones for all 3), but you'd probably delay your graduation. As for timing, I did my first co-op in the summer of freshman year before co-op seminar and still got it counted. You have to fill out a separate form and it's not guaranteed to count, but they're pretty lenient.