I participated in the Album Challenge this past year and loved it so much! Didn't get it done before the deadline (actually I'm still working on the last two fills because I'm ridiculously slow at writing 😅) but it's easily been one of my favorite fic challenges in a long while. Would love to see it return again this year.

I can't really speak on the other events since I got too busy during that time to participate.

As for the number of events, I think what you're doing with three per year is good. It allows them to be spaced out nicely and prevents overwhelm for anyone who is participating in more than one fic challenge community.