Planning for reference letters, what should I do?

I’m a first year undergrad studying astronomy/physics and I’m trying to plan ahead for grad school (PhD). I’m currently working in a research group with a supervisor I really like, and ideally I’d want to stay with them for all four years.

The issue is reference letters. From what I understand, I’ll need around 3 strong letters, and I’ll definitely have one from my current supervisor. I’m also thinking I could get another from a summer program after third year.

But that still leaves me needing one more strong letter, and I’m not sure how to get it without leaving my current research group.

I don’t really want to switch groups just for the sake of getting another letter, since I’d prefer to stay and build a deeper, long term project where I am. At the same time, I don’t want to end up with weak or redundant letters.

For people who stayed with one main supervisor during undergrad, how did you handle getting multiple strong reference letters? Are letters from course professors actually taken seriously in physics/astronomy? Or is it better to try to get involved in a smaller side project with someone else?

Would really appreciate any advice, especially from people who’ve gone through physics/astro PhD applications.