In search of a way to curb the Southeastern Conference’s swelling power, three of the wealthiest college sports leagues — the Atlantic Coast, Big Ten and Pac-12 conferences — said Tuesday that they had formed a coalition to navigate a turbulent era.

Beyond a “scheduling alliance” that could eventually produce more marquee matchups in basketball and football, the three conferences could as soon as next month upend a proposal to triple the size of the College Football Playoff to 12 teams. The conferences could also prove a crucial force as the chronically embattled N.C.A.A. contemplates a reorganization.