There is no recognised word for it. Although it would be possible to construct one using Greek (or Latin) prefices, and that would be good for obfuscation or being pretentious, it would not be much use if you wanted to communicate. I would suggest "eighteen-fold".
18 / (9*3) = 0.66666.... (18/9) * 3 = 6.
The diagonal of an 18 inch square is about 25.46 inches. (18 times the square root of 2).
18 ft Diagonal of a square is the square root of 2, times the side.
3 times the square root of 2
3 sqrt(6) x sqrt(6) = 18
quintuplicate sextuplicate septuplicate octuplicate nonuplicate decuplicate
quintuplicate sextuplicate septuplicate octuplicate nonuplicate decuplicate
Triplicate. eg This letter should be done in triplicate.
"triplicate"
triplicate
Writing in triplicate.
You write something out in triplicate.
Writing in triplicate.
triplicate
The future tense is "will triplicate".
Performing a measurement or an experiment three-times is called a triplicate. Triplicate results make statistical analysis better and prevent the possibility of unusual results due to natural/artificial variation.
The form must be filled out in triplicate.