We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution - we find 26 stars with v_rf>275 km s-1 and 1 star with v_rf<-275 km s-1 - shows that the HVSs must be short-lived, probably 3 - 4 M_\sun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 M_\sun HVSs fill our survey volume; shorter-lived 4 M_\sun HVSs are missing at faint magnitudes. We infer that there are 96+/-10 HVSs of mass 3 - 4 M_\sun within R<100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a 120 Myr-old burst of HVSs is also allowed.