This could represent the Holy Trinity in it's simplest form and at the same time excluding secret orders formed hundreds of years ago. My estimate of age mid to late 19th century, judging from the retainer clip on the back and the apparent depth of oxidation but that's just my opinion.
Definitely a belt-buckle, and most likely not a Military one. Probably in the "Fraternal Organization" category. Many such organizations used a triange in their symbolism.
Your photos seem to show no sign of a rectangular belt-attachment bar being broken off the side of the buckle... so, apparently, it is missing the brass belt-attachment "bar" that was soldered onto its back (which is why the buckle's owner discarded it).
Accurate size-measurements would be helpful. You didn't provide any measurements, but going by what the photo shows, unless you've got really short fingers the buckle is smaller than most buckles of that type.
A "rule of thumb" about time-dating that type of rectangular belt-buckle is the width of the tongue-shaped hook on the back. A narrow tongue-hook, like we see on yours, tends to be "earlier" than wider tongue-hooks. Based on that rule-of-thumb, your buckle PROBABLY dates from the mid-1800s to late-1800s, instead of the 20th Century. I lean to it being late-1800s, because it is a solid-cast (rather than stamped-brass) buckle whose belt-attachment bar was on its back instead its side.