Triad Paperbacks Ltd was a joint paperback imprint of Chatto, Bodley Head and Jonathan Cape and Granada Publishing Ltd. As several of these were effectively the same company at the time, it seems likely that "Triad" came from the three imprints used in combination with "Triad".
The books themselves usually have "Triad" in combination with the name of another paperback imprint from one of the owners on the spine. This may differ from what is stated on the front cover, the Copyright page or Title page: e.g. "Triad Granada" may say "Triad Panther" internally, published by "Granada Publishing Ltd".
Which combinations are actually seen in the wild is not yet known, but we currently have records for Triad in combination with Grafton, Granada and Panther (known paperback imprints) and it's unlikely that Triad was ever used in combination with a hardback imprint like Chatto, Bodley Head or Jonathan Cape.