v3 should be able load any data from v2, but v2 can't load data from v3. In some instances, where file formats have not changed (most did), v2 *can* load a v3 file. But even then, this is really not a supported feature.
Version 2.5.14 did include the ability to load a player database updated by v3. This was intended to help folks recover from an updated database when they downloaded and ran v3 to test it out. If a database backup wasn't made, it was basically lost if the person decided to continue to use v2, and this was happening quite a bit.