Threefold structures occur across enough unrelated traditions that the interesting question is not whether they exist but how differently they work.
In Christianity the triangle is the standard visual shorthand for the Trinity, most explicitly in the medieval diagram called the Scutum Fidei or shield of faith, which sets Father, Son and Spirit at three corners with God at the centre and labels the connecting bars to state precisely which identities hold and which do not. It is a piece of doctrinal engineering as much as an image, designed to head off the errors the councils had spent centuries naming. The eye set within a triangle, common on eighteenth-century church furnishings and later on Masonic and civic emblems, is a related but much looser use.
In Hindu tantra the upward triangle is read as Shiva and the downward as Shakti, and the Sri Yantra is built from nine interlocking triangles, four upward and five downward, converging on the central bindu. The reading is directional and gendered in a way the Christian use is not.
Egyptian religion grouped gods in threes, Amun, Mut and Khonsu at Thebes, or Osiris, Isis and Horus, and Egyptologists generally read these as family or local groupings rather than as any statement about unity within plurality. Reading them as a proto-Trinity is a nineteenth-century habit that the evidence does not support.
Alchemical and Western esoteric traditions used the triangle as the elemental sign: point up for fire, point down for water, each with a horizontal bar added for air and earth respectively. That scheme is medieval and early modern rather than ancient, though it is frequently presented as far older than it is.