  We report the discovery of a Jupiter-size planet transiting a relatively
bright (V = 11.56) and metal-rich early K dwarf star with a period of about 2.9
days. On the basis of follow-up photometry and spectroscopy we determine the
mass and radius of the planet, HAT-P-3b, to be M_p = 0.599 +/- 0.026 M_Jup and
R_p = 0.890 +/- 0.046 R_Jup. The relatively small size of the object for its
mass implies the presence of about 75 Earth masses worth of heavy elements (1/3
of the total mass) based on current theories of irradiated extrasolar giant
planets, similar to the mass of the core inferred for the transiting planet HD
149026b. The bulk density of HAT-P-3b is found to be rho_p = 1.06 +/- 0.17
g/cm^3, and the planet orbits the star at a distance of 0.03894 AU. Ephemerides
for the transit centers are T_c = 2,454,218.7594 +/- 0.0029 + N (2.899703 +/-
0.000054) (HJD).
