If your in an all U.S.A. market and an all world ETF and total bond ETF then there really isn't much more you can do to be diversified. Using more ETF's from different fund companies won't make you more diversified because they will be invested in the same things.
I don't use the three fund method. I have Large cap, Mid cap, Small cap blend funds. I have REITs, health care and utilites and some international too.