I am applying to a graduate program in computer science. One of the requirements for applying to this graduate program is to have 3 recommenders write a letter of recommendation for me. I have been working on a hobby project in my spare time. This hobby project has a sizable number of people working on it (~15 people) and a very significant amount of work has been done on it. I am a 'team lead' on this project as well. This project also has a single guy in charge of it that could speak of the work I've done on it. How appropriate would this 'head' of the aforementioned project be as someone who writes a letter of recommendation for me?
You want your letters to come from people who can attest to your ability to perform in whatever you are applying for.
For graduate school, admissions committees are going to get the most insight from recommendations coming from people who both know you and also know what a MS in CS requires. Ideally, that means people who teach masters level CS courses and are familiar with students who do well and do not do well. In other words, professors.
Other people, like a boss outside of academia, can also round out your application, but you don't want to rely on them primarily.
The particular example you give seems rather weak. Unless your hobby project is something well-known or for some cause that might impress people, I'd wonder why that was the best letter writer you could find. It seems closer to asking for a recommendation from a friend than a boss.
I'd probably look for other options.