hybrid

Three-year-old wagon for $15k? This is a tough one, as the only offered recently in your price range is the Volkswagen Jetta SportWagen. With a diesel the VW is very efficient, and with any engine it handles pretty well. Reliability is so-so.

The new 2015+ Golf SportWagen will likely be more reliable, and handles better, but unless dealers are discounting them massively (which is a possibility) you'll have trouble finding one for $15k. Worth checking.

If you can spend a little more and/or go a little older, you might be able to find an Acura TSX wagon or a Subaru Outback in your price range. The Subaru can hold the most of the three, and the Acura the least.

One not-quite-wagon that just crossed my mind is the Ford C-MAX. The 2013 had some problems, but the 2014 has been reliable so far. Limited cargo capacity, but good fuel economy since it's a hybrid.

The Toyota Prius v has more cargo capacity than the C-MAX, and has stellar reliability, and also might be a little more fuel efficient, but it doesn't drive nearly as well. The largest problem with the Toyota, though, is it costs quite a bit more as a used car, since Toyotas depreciate more slowly than Fords, so you probably won't find one near your maximum.