A three-fund portfolio is based on the fundamental asset classes, stocks and bonds. It is assumed that cash is not counted within the investment portfolio, so it is not included. On the other hand, it is assumed that every investor should hold both domestic and international stocks. The task, then, is to take these three basic non-cash assets — domestic stocks, international stocks, and bonds — decide how much of each to hold (your asset allocation); choose where to hold each of these asset classes, and finally choose a mutual fund to use for each asset class.
Suggest an allocation to implement a three portfolio strategy as described in the text below
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.