From Vanguard's list of "core funds," the funds that are best for a three-fund portfolio are:

Vanguard Total Stock Market Index Fund (VTSAX)
Vanguard Total International Stock Index Fund (VTIAX)
Vanguard Total Bond Market Fund (VBTLX)
So, a "three-fund portfolio" might consist of 42% Total Stock Market Index, 18% Total International Stock Index, and 40% Total Bond Market fund.