I think it’s great that you are clarifying your goals and researching possibilities to help you meet your goals. A few points though
“my end goal is a Master of Science in Mechatronics offered by the university that requires previous study in either ME, EE, and/or CE”
I would sniff around the department for Mechatronics at your U and try to clarify what the above requirement means specifically. I doubt most applicants are triple majors; most will have a single major with a concentration of course work in another discipline. Get a sense regarding additional coursework the department values. Maybe they like to see specific upper level CS courses?
Don’t discount the value of real world experience through research or internships. Professors like to know you can work on team, deliver a presentation, write up results etc. Those skills are more easily demonstrated in the real world environment of the lab than during an academic course. Also, by working with a PI (principal investigator), you can obtain a recommendation, crucial for grad school. Of course, many of these opportunities will consume time during the summer.
It sounds like you really want to augment your studies in your principal major with classes in other engineering disciplines. I would pick those classes based on your passions and in areas where you want to develop your skills. Remember, applying from college to grad school is much more like applying to a job than the high school to college admissions process. Professors are looking for grad students to work for them in order to further their research. Developing a specific area of research interest will help you appeal to professors working in that area of the discipline. You might want to start by browsing through the Mechatronics faculty, reading their journal articles, and learning more about their areas of expertise.
Looking into the summer sessions and potential classes offered is a great idea. One thing to be aware of-- sometimes the faculty teaching summer classes are not the same as the people who teach in the regular fall and spring semesters. My son’s U has a few legendary profs teaching core courses for his major during the regular year, but during the summer, these courses are taught by other professors. Of course, depending on the situation, it’s not always a negative.
What an exciting journey you are beginning!