“third hour.” About our 9 AM. The original workers had likely started about 6 AM, the start of the day, and when hired for a day were expected to work until 6 PM; 12 hours.
Both the Jews and Romans divided the day into 12 hours, starting at daylight, roughly 6 AM. The start of the Christian Church on the Day of Pentecost, marked by the outpouring of the gift of holy spirit, occurred at the third hour of the day (cp. Acts 2:15). [For the hours of the day and the watches of the night, see commentary on Mark 6:48].