Triangles, specifically, can be used to represent any number of themes; be they religious (such as the holy trinity) or mythological (such as Norse folklore).
In regards to the particular subject matters mentioned, a simple triangle with a cross inside would be a fairly understated way to represent such beliefs.
The implied “father, son, and holy spirit” idea is adequately accounted for with the three sides of the figure, so there is no need to be more obvious than that.
It is entirely possible to be too “clever” with one’s tattoo, the meaning being well and truly lost in the process; however, it is not good to be too obvious either.
In terms of Norse mythology, the Valknut features three interlocking triangles and is thought to have the ability to cast magical spells. Each one of the triangles could be rendered in a different color, or perhaps created in basic black.
Either way, you can cast your own bit of magic with one of these tattoos in a well-chosen and visible spot, immediately mesmerizing anyone who gazes upon it. The person standing behind you in line is a prime candidate (if you put it on the back of your neck and have your hair up)!
Similarly, triangles can be used to create M.C. Escher/optical illusion type patterns that will surely intrigue and confound the observer. Making someone go cross-eyed staring at your tattoo seems a bit cruel, but so it goes.
A triangle design can also serve as a frame that features an image inside. For example, one’s astrological sign can be housed in the triangle. See more cool geometric ideas for your next bit of ink below!