I really shouldn't nitpick (so please do forgive me), but strictly speaking the whole company doesn't manage $3T. It is true that's the best number to compare to the rest of the industry, but Vanguard has a rather unique corporate structure. The end result is that each Vanguard fund is a company which owns part of the umbrella group. This inverted corporate structure is set up so that the fund-owners have direct stake in management 1) of the umbrella organization 2) only of each individual fund, not other "leaves" of the tree.
It might seem like a distinction without a difference, but it's the single greatest reason their cost structure is so much more attractive.