I'm not sure what you're doing, but here are two ways to approach this:

The chance of rolling at least 1 tripple 6 is:
1/216+215/216*(1/216+215/216*(1/216+215/216*(1/216)))

Alternatively, the chance of not rolling any tripple sixes will be
(215/216)^4
so the chance of rolling at least one, must be
1-(215/216)^4