I think KakoxBocchixSotoka probably works the best.
These two obviously work together and she's not actually that close to kei-chan. Sotoka and Kako were the two with the biggest single emotional influence from bocchi outside of kou, the two with the most emotional influence from bocchi, both sought out special relationships with bocchi that they then sought to modify to get closer to her, and are the two that have the most respect for bocchi. They also get along really well together, so it seems like a stable triad.