Three or Six Hours on the Cross?

A blessed Good Friday to you all. I have found that many devotional manuals and tracts concerning the death of Our Lord speak of his “three hours” on the cross.

However, it seems clear that he was on the cross for six hours, from the third hour (9am) till the ninth hour (3pm).

Mark 15:25
25 And it was the third hour (9am), and they crucified him.

Mark 15:33-37
33 And when the sixth hour (i.e. 12pm) was come, there was darkness over the whole land until the ninth hour.
34 And at the ninth hour (i.e. 3pm) Jesus cried with a loud voice, Eloi, Eloi, lama sabachthani? which is, being interpreted, My God, my God, why hast thou forsaken me?
35 And some of them that stood by, when they heard it, said, Behold, he calleth Elijah.
36 And one ran, and filling a sponge full of vinegar, put it on a reed, and gave him to drink, saying, Let be; let us see whether Elijah cometh to take him down.
37 And Jesus uttered a loud voice, and gave up the ghost.

Given the Scriptural evidence, from what sources does the “three hours” tradition derive?

A blessed Good Friday to you all. I have found that many devotional manuals and tracts concerning the death of Our Lord speak of his “three hours” on the cross.

However, it seems clear that he was on the cross for six hours, from the third hour (9am) till the ninth hour (3pm).

Mark 15:25
25 And it was the third hour (9am), and they crucified him.

Mark 15:33-37
33 And when the sixth hour (i.e. 12pm) was come, there was darkness over the whole land until the ninth hour.
34 And at the ninth hour (i.e. 3pm) Jesus cried with a loud voice, Eloi, Eloi, lama sabachthani? which is, being interpreted, My God, my God, why hast thou forsaken me?
35 And some of them that stood by, when they heard it, said, Behold, he calleth Elijah.
36 And one ran, and filling a sponge full of vinegar, put it on a reed, and gave him to drink, saying, Let be; let us see whether Elijah cometh to take him down.
37 And Jesus uttered a loud voice, and gave up the ghost.

Given the Scriptural evidence, from what sources does the “three hours” tradition derive?