Current grad student and 3-sport D1 athlete here (xc, indoor/outdoor t&f) so I've been in-season year-round for the past 5 years, excluding a year I had to take off from competition due to illness which is why I'm still eligible this year. I finished undergrad with a 3.97 GPA in bio (BS) with two minors in 7 semesters, so it's definitely possible. Here's a couple pointers I've picked up along the way; I hope they help!
1) Embrace being the team nerd. You're going to have to study on those never-ending bus rides. You're going to have to skip some team parties and you're going to go straight from practice to the library or the lab. Your teammates with aspirations closer to the mantra of "C's get degrees" will not be doing this. Learn how to endear yourself to the team culture without compromising your academics. Making friends outside of the team helps a lot, too!
2) SLEEP. Oh my god, sleep. Get your 8 hours whenever you can. You'll perform better athletically and academically, and you're much less likely to burn out.
3) Start volunteering early - you're not going to be able to get in more than a few hours a week, so make sure they add up in the end. I volunteered at a local hospital working one four-hour shift per week which was totally manageable, but I did it for two years so I ended up with close to 250 hours. Med schools also value long-term commitment, so double-whammy!
4) Make sure your professors know your face. Be "that guy" in the front row on the first day. Introduce yourself and tell them about your athletic commitment. Let them know when you'll be missing class for competitions; deliver class-excuse forms in person if you can and ask what you can do to make up missed material. A lot of profs are used to athletes that expect the professor to pick up the slack instead of the student-athlete; show them that's not you and they'll usually be more than willing to work with you.
5) See if there's any community service opportunities that you can participate in through athletics - my school has a club for student athletes interested in community service, and they do some pretty cool things. It's way easier to get involved, and they're more catered towards a typical athlete's schedule.
6) Never took any summer courses so I can't comment on those (can't imagine it would look bad if you got good grades though), but I highly recommend using summers to go hard on those ECs... research internship if you can nose your way in, summer camps with at-risk/special needs youth, shadowing, etc. Definitely pick one or two additional ECs to keep up during the school year if you can, but it'll be hard to manage more than that with your schedule. Summers are great to get those hours in!
7) Embrace the lifestyle. It's nuts. I've taken a final exam proctored by my assistant coach on the infield of a indoor track while eating greasy pizza at 10 PM. I've gotten back from a 3-day championship meet at 2:30 AM and had an exam at 8 AM. I've had night-lab partners cover for me while I took a break to go scarf down some dinner in the hallway because I came straight from practice. Learn how to laugh at how ridiculous it is instead of crying over it (it's hard to study through tears), and how to get tough instead of making excuses (this was a HUGE attitude shift for me). Make sure you love what you're doing, and you'll be just fine. It's hard, but I promise it's worth it! I'm one semester away from blessed retirement, but I can tell you I wouldn't change a thing.