The DP-3 is a better finder, if you plan to use non-AI lenses with full-aperture metering. Otherwise, the DP-12 is better. The only two DP-3's I've dealt with have either had broken FRE's, causing one indication to be displayed at all times, or were quite erratic, whereas, the multiple DP-12's I've owned have all been smooth in operation and accurate. Even the two finders that had impact damage.
Of the two, the DP-12 is a lot more common than the DP-3, since, like Dia mentioned in his post above mine, it was a 1976-77 finder, whereas the DP-12 went from 1977-early 1980. So, a body with a DP-3 (F2SB) will be more expensive than the body with a DP-12 (F2AS). Surprisingly, the F2's with the DE-1 nonmetered prism are all over the place, price-wise.