  We present a sample of 407 z~3 Lyman break galaxies (LBGs) to a limiting
isophotal u-band magnitude of 27.6 mag in the Hubble Ultra Deep Field (UDF).
The LBGs are selected using a combination of photometric redshifts and the
u-band drop-out technique enabled by the introduction of an extremely deep
u-band image obtained with the Keck I telescope and the blue channel of the
LRIS spectrometer. The Keck u-band image, totaling 9 hours of integration time,
has a one sigma depth of 30.7 mag arcsec^-2, making it one of the most
sensitive u-band images ever obtained. The u-band image also substantially
improves the accuracy of photometric redshift measurements of ~50% of the z~3
Lyman break galaxies, significantly reducing the traditional degeneracy of
colors between z~3 and z~0.2 galaxies. This sample provides the most sensitive,
high-resolution multi-filter imaging of reliably identified z~3 LBGs for
morphological studies of galaxy formation and evolution and the star formation
efficiency of gas at high redshift.
