We report the discovery of a very bright ( m _ { R } = 22.2 ) Lyman break galaxy at z = 3.03 that appears to be a massive system in a late stage of merging .
Deep imaging reveals multiple peaks in the brightness profile with angular separations of \sim 0. ^ { \prime \prime } 8 ( \sim 25 h ^ { -1 } kpc comoving ) .
In addition , high signal-to-noise ratio rest-frame UV spectroscopy shows evidence for \sim 5 components based on stellar photospheric and ISM absorption lines with a velocity dispersion of \sigma \sim 460 km s ^ { -1 } for the three strongest components .
Both the dynamics and high luminosity , as well as our analysis of a \Lambda CDM numerical simulation , suggest a very massive system with halo mass M \sim 10 ^ { 13 } M _ { \odot } .
The simulation finds that all halos at z = 3 of this mass contain sub-halos in agreement with the properties of these observed components and that such systems typically evolve into M \sim 10 ^ { 14 } M _ { \odot } halos in groups and clusters by z = 0 .
This discovery provides a rare opportunity to study the properties and individual components of z \sim 3 systems that are likely to be the progenitors to brightest cluster galaxies .