User blog comment:CaveJohnson/I just realized.../@comment-25028958-20151005120805/@comment-4554577-20151005184938

In Christianity and perhaps many more religeons, three sixes is related to the Devil/Hell.

The Nightmare Realm appear's to be the Gravity Falls equilivant of the Underworld/Hell.

Paticurally in Christianity, having three sixes tatooed onto you means you 'belong to the Devil'

and there are also some people who say writing or maybe saying this number aloud will summon The Devil himself.

so three six fingered hands open a portal, one Bill wanted to open to the Nightmare Realm, which again, is basically hell itself.