Thanks a lot for the thoughts, everyone! Given all the unknowns, it's seems to be a speculatively interesting question. I'm still not quite sure what to do. The good thing is, there's no pressing need to change anything. As for next steps, I just opened a Vanguard account and put 3k into the Total Stock Market and am looking to start making monthly contributions.
I'll also be calling my mortgage person and asking what kind of refinance options there are, just to run the numbers. Maybe if I can get a 3% loan, and take out just a little bit of cash, then it would be a good move in the name of diversification.
To answer some of the Questions: One of the houses is in Portland, Oregon, and the other two are in Bend, OR. Both are solidly appreciating markets, but there is the 1/3 chance that a big earthquake will hit in the next 50 years, making the equation even more interesting :)