The evidence for this symbol is entirely archaeological. No text from the Norse world mentions it, describes it, or gives it a name.
What survives is a small set of objects. On Gotland picture stones, notably Stora Hammars I, the figure occupies a prominent position within a scene that also shows a spear, a hanged man, a bird and what is generally read as a burial mound. It recurs on the Tangelgarda stone, there drawn in unicursal form, below a horse. It is carved on a bedpost from the Oseberg ship burial in Norway and appears on tapestry fragments from the same find. It is engraved on a gold finger ring from the Nene valley in England, dated to the eighth or ninth century. A seventh-century coin found near Norwich carries both the figure and a Christian cross, which is a useful corrective to anyone treating it as a confessional badge.
Two geometrically distinct forms exist: three separate interlocking triangles, and a single continuous line folded into the same outline. Whether the makers regarded these as the same sign is unknown.
That is the whole corpus, and it is worth registering how thin it is: a handful of objects, spread across Sweden, Norway and England, over perhaps three centuries, none of them captioned. The find contexts are also mixed in a way that resists a single explanation, since a bed from a rich woman's burial and a gold ring worn on a hand in Anglo-Saxon England are not obviously the same kind of statement.
Everything beyond the objects, what it meant, whether it had a name, whether the people who cut it into a bedpost and the people who cut it into a memorial stone meant the same thing at all, is inference from context, and the context is itself being interpreted.