Actually the D + D reaction has two equally probable branches (and one very improbable one that makes an alpha particle and a gamma ray, which is mostly ignored), one makes Tritium and a Proton and the other makes D + He3. So the D + D reaction will breed He3 fuel, along with Tritium. The resulting proton from the Tritium branch can also combine with Deuterium and create Helium 3 as well in the right conditions. Under carefully control, the right conditions, and with some engineering work I would wager you could bias the reactions and side reactions towards creating more Helium 3.