The hour has a special meaning in Jewish law. "The third hour of the day" doesn't mean 3:00 a.m., or three sixty-minute hours after sunrise. Rather, an hour in halacha is calculated by taking the total time of daylight of a particular day, from sunrise until sunset, and dividing it into twelve equal parts. A halachic hour is thus known as a sha'ah zemanit, or proportional hour, and varies by the season and even by the day.
For example, on a day when the sun rises at 5 a.m. and sets at 7:30 p.m., one sha'ah zemanit, or proportional hour, will be 72.5 minutes long. The third hour of the day will come to a close at 8:37:30 a.m.
This information is important because many observances in Jewish law are performed at specific times during the day. The calculation of these halachic times, known as zmanim ("times"), depends on the length of the daylight hours in that locale.
For more information regarding the various halachic times of the day, as well as some of their associated mitzvot, see About Zmanim.
To find out the halachic times for any location, see Zmanim-Halachic Times.