Is the number triple-6 good, bad, or neutral in Judaism?
How about numeric variations and combinations such as 216?
I know that 18 means 'life' due to Gematria calculations.
This number has, to the best of my knowledge, no significance whatsoever in Judaism.
You can search according to gematria here but, as my teacher Rabbi Tzvi Aryeh Rosenfeld z'l said in a shiur, the average person shouldn't really come up with their own gematrios, generally speaking, and expect anything from them.
You will find that there are many gematrios with both good and bad implications for practically every value.
666 is very significant in two locations in Genesis.
First, in the beginning of creation, the values for light, dark, day and night add together into the sum 666. אור=207, חשך=328,יום=56, לילה=75. Though חשך commonly is written with a ״ו״ (vav), in the creation account it isn't.
Second, when Joseph becomes second in command of Egypt, Pharaoh renames him צפנת פנעח. The value of this name is 828. He demands Benjamin come down to Egypt. בנימין has a value of 162. 828-162=666. In regard to what 666 is in creation, it then fits in context of the Joseph narrative and the fulfillment of his final dream before his brothers threw him into the pit.