There was no 'mark three' because the objective in measuring the water depth, was simply to make sure it was deep enough to prevent the boats from running aground. Two fathoms is about 12 feet, which was deep enough for most any riverboat. Their hulls weren't very deep, so they could easily navigate rivers.

"Mark Twain" (meaning "Mark number two") was a Mississippi River term: the second mark on the line that measured depth signified two fathoms, or twelve feet—safe depth for the steamboat.

That said, treble may be used as an attributive adjective meaning: Consisting of three parts; threefold (Oxford Dictionaries). So you could say:

• I would like to split something treble.
• I will cleave this rock treble.

There was no 'mark three' because the objective in measuring the water depth, was simply to make sure it was deep enough to prevent the boats from running aground. Two fathoms is about 12 feet, which was deep enough for most any riverboat. Their hulls weren't very deep, so they could easily navigate rivers.

"Mark Twain" (meaning "Mark number two") was a Mississippi River term: the second mark on the line that measured depth signified two fathoms, or twelve feet—safe depth for the steamboat.

That said, treble may be used as an attributive adjective, meaning: Consisting of three parts; threefold (Oxford Dictionaries). So you could say:

• I would like to split something treble.
• I will cleave this rock treble.