Probable transcription factor involved in the regulation of flowering time under short day (SD) conditions. Functions as repressor of flowering under SD conditions, independently of HD1, EHD1, MADS50 and MADS51. Controls flowering time under SD conditions by negatively regulating the expression of HD3A and FTL. Circadian-regulation under long day (LD) conditions. Expression increases after the beginning of the dark period, peaks at dawn, stays high four hours and gradually decreases until sunset. Plants over-expressing CO3 display late flowering under short day (SD) conditions. Belongs to the CONSTANS family.