I would like to purchase a few chestnut trees as an anniversary gift. I have read they need to be purchased in two's to promote pollination. Does that mean I should not order only three? I wanted them to 'represent' our family unit.
The question you pose raises a couple of questions. The genus of native Castanea trees are nearly extinct so you will need Asian species. Is that what you meant by Chestnut or an do you want another tree? Secondly I don't believe they are adapted to USDA zone 10 and wouldn't thrive. It's usually better to purchase pairs of nut trees unless you know there are existing ones to pollinize the crop.
I'm interested in the Dunstan Chestnut trees to help bring deer to the property. They sell for $24.95 each online.
That still leaves the question about your USDA zone which it too warm for proper dormancy of the trees in Winter. Is this an issue for you? You will need a second tree (possibly Asian for pollination). Looks like blight is an issue for this Asian Hybrid Chestnut. http://forums2.gardenweb.com/forums/load/trees/msg0816584123195.html
Further reading turns up that these are seed grown and can't really be the named variety! If that's true in your case three is fine as one seedling can always pollinize another. The blight resistance in unlikely to transfer from the parent tree to the offspring. Good luck.