3 Geminorum is a blue supergiant star in the constellation Gemini. It is a small amplitude pulsating variable and a close double star, with a mean combined apparent visual magnitude of 5.75.
3 Geminorum was found to be an α Cygni variable in 1998 and given the designation PU Geminorum. It varies by a few tenths of a magnitude with a main period of 3.81 days.
3 Geminorum is also a close double star. The brighter component is the variable blue supergiant. The companion is 2.5 magnitudes fainter. The separation is about 0.6 arc-seconds. There is also a much fainter, approximately 14th magnitude, star 14" away.
Faint Hα emission lines have been detected in the spectrum of 3 Geminorum, but this is not usually expressed in published spectral classifications. An "e" is only occasionally appended to the spectral type to reflect the emission lines. MK spectral types consistently classify 3 Geminorum as a normal supergiant (luminosity class Ib), although spectral classes derived in other ways often result in a bright supergiant (Ia) luminosity class.
3 Geminorum can be occulted by the moon. Observations of these occulations can give information about the angular diameter of a star, or about close companions. Occultations of 3 Geminorum have been observed, but no double or diameter information has been published.