What is Triple-stranded (Helix) DNA?

"The DNA double helix can under certain conditions accommodate a third strand in its major groove."
https://www.chemistryworld.com/news/6-june-2005-a-third-strand-for-dna/3000164.article

"The distinctive three finger grasp represents the transformed Beast DNA of a triple helix construction."
http://www.theopenscroll.com/ashWednesday.htm

"A triple helix is named such because it is made up of three separate helices. Each of these helices shares the same axis, but they do not take up the same space because each helix is translated angularly around the axis. Generally, the identity of a triple helix depends on the type of helices that make it up. For example: a triple helix made of three strands of collagen protein is a collagen triple helix, and a triple helix made of three strands of DNA is a DNA triple helix."
https://en.wikipedia.org/wiki/Triple_helix

"Triple-stranded DNA structures were common hypotheses in the 1950s when scientists were struggling to discover DNA's true structural form. Watson and Crick (who later won the Nobel Prize for their double-helix model) originally considered a triple-helix model, as did Pauling and Corey, who published a proposal for their triple-helix model in 1953,[19][20] as well as fellow scientist Fraser"
https://en.wikipedia.org/wiki/Triple-stranded_DNA

"A technique to insert a third strand of DNA into the double helix of
specific genes could be used to throw a spanner into the works of the cell’s
machinery. The American scientists who developed this triple helix, or triplex,
technique hope to eventually cripple viruses in this way. The same approach
could also inhibit the hormone progesterone, and so terminate pregnancies"
https://www.newscientist.com/article/mg13017644-100-science-a-triple-helix-to-cripple-viruses/