Three friends come together, who once had decided not to meet again. They get together to find their fourth friend.

This isn't really a logline, it's a situation.

That being said, I think it's an interesting premise. Three friends, who were never planning on meeting again, coming together to find their missing friend. You need to be specific though. Is their 4th friend missing? Is that the inciting incident? We need to follow one of these three as the protagonist so write it from their perspective.

We also need to understand why this is so difficult. What obstacles are standing in their way? Who's trying to stop them? We need an antagonist, and we need stakes. What will happen if they don't find him? Is he in danger?

Surely the title is "And then they were three" since they've lost one?