If you aren't familiar with the 3 fund portfolio, the summary is that when investing in stocks, you should only be buying total market index funds. The 3 index funds represent: total US, total non-us (i.e. International) and total bonds.
The intent is to get the lowest fees, since that's basically the only part of investing that you have total control over. In reading about this in Elements of Investing, it appeared that Vanguard is usually the lowest fee alternative. But I ran the numbers on the funds suggested at the Bogleheads 3 fund portfolio page on this FINRA fee calculator, and it appears that the iShares ETF beat all the mutual funds every time.
If you aren't investing in Admiral class shares at Vanguard (i.e. over 10k) then you won't beat the ETF. If you do qualify for Admiral Shares, you will slightly beat the ETF fees.
If your money is already at a specific brokerage (Fidelity, Vanguard, Chase, etc) why would you buy that brokerages index mutual fund, as opposed to just investing in the iShares ETF?