What makes a 3 person group special?

Intuitively I feel like 3 person groups tend to be more efficient than 1 person, 2 person, or 4+ person groups. I can't rationally explain this intuition, maybe you can help understand this?

I'm looking at this from a productivity perspective. Think business founders, military strategists, political strategists, etc.

From what I currently understand:

1 person

- Narrow perspectives
- Flaws caused by individuality aren't covered for
- Can literally get less done because there is 2-3x less brainpower than other groups.

2 people
- Maybe a sort of group-think can develop?
- you become to similar to each other?
- Easily avoid bad strategic decisions because you don't move forward unless both agree.

3 people.
- Far less likely for there to be groupthink,
- still a sense of closeness to each other, and ideas
- still a significant increase in brainpower from 2 people

4+ people.
- Feels significantly less close & like the efficacy of each individuals brain power rapidly falls compared to smaller groups
- Intuitively it feels to me like productivity begins to rapidly fall as you get to 4+ people
- I can't tell you why in a rational way because I don't know why I subconsciously believe this.