 Transits in front of the star GSC 03089-00929 were detected by two 10 cm telescopes operating out of Palomar and Lowell Observatories.Transits of TrES-3 were also independently observed by the Hungarian Automated Telescope Network.Seven high-precision radial velocity measurements were made using the HIRES spectrometer at W. M. Keck Observatory, which confirmed the presence of a planetary mass companion with a minimum mass twice that of the planet Jupiter.