A ter'angreal in the shape of three rounded, silver arches, just tall enough to walk under, sitting on a thick silver ring with their ends touching each other. It was found during the Trolloc Wars. It sits in one of the lowest levels of the White Tower and is used to test a novice who is being promoted to Accepted. The three arches present visions related to one's past, present and future fears. It is not clear if the visions have any relationship to reality.
There is a second, similar ter'angreal in Rhuidean in the shape of three touching rings. It is part of the testing for becoming an AielWise One. The three rings in Rhuidean show possible futures.
TGH,Ch23 - Nynaeve passes through the three arches where she meets Aginor in a stone maze, she sees Emond's Field falling apart without her and she sees herself married to Lan.
TDR,Ch23 - Egwene barely makes it back from the three arches. The twisted ring resonates with the three arches causing it to malfunction and nearly destroy itself.
TFoH,Ch7 - Now that Rhuidean is populated again, the three rings are fenced off to prevent anyone from accidentally entering.
TFoH,Ch7 - The three rings show a woman all her possible futures based on decisions she will make. Most of these memories fade until the moment of decision is upon her.
TFoH,Ch31 - Aviendha learned of her romantic involvement with Rand in the three rings.
CoT,Ch10 - From her trip through the three rings, Aviendha has only vague memories of her future such as falling in love with Rand and having sister-wives. Nevertheless, something from her trip tells her that going to Rand now would lead to disaster.
ToM,Ch49 - After her trip through the glass columns, Aviendha remembers some things from her trip through the three rings. She remembers her children, but she does not remember these visions of the future.