Human beings love the number 3. There are three sides to a triangle, the most stable of all geometric shapes (this is why trusses are made of triangles). In many of the earliest known languages, it’s the highest named number. Basically, whether we’re dividing things into a beginning, a middle, and an end or grouping Egyptian deities, as long as we’ve been organizing things, we’ve gravitated toward the number 3.
Even when it doesn’t make sense.
In recent years, a vocal subset of Celtics fans have spent off-seasons constructing imaginary future rosters, and these rosters tend to congeal around the idea that the C’s need to have a “Big Three” in order to contend for a title.