It sounds like the role of bonds in a three-fund portfolio is stability, and to provide a separate asset class for rebalancing purposes. The PP should theoretically be more stable than bonds, since it is designed specifically to weather all economic conditions. It is also not correlated with stocks, so it should be just as suitable as bonds for rebalancing.
The only downside I can think of is that the ER will be higher due to the gold, but if some of the gold is held as ETF's it will cut back on most of the bullion transaction costs. The added expense over a single total bond fund shouldn't be too significant.
Does this make sense?