Taken from the tales of King Aurther and his knight, Percival, this refers to a person or couple who is searching for the "holy grail", the "unicorn", the HBB.
Percival did locate the Grail at one time, but, being too immature and failing to ask the proper question, he failed in attaining the Grail and he must grow spiritually and mentally before he could locate it again.
Percival is described in some places as "the least worldly and the least groomed of all the knights", having been raised in the woods away from society, and "extremely pious but somewhat naive".
This is analogous to a couple who is new to polyamory and, lacking sufficient role models in alternative relationships and lacking the experience themselves to understand the complex nature of multi-partner relationships, the couple believes, rather naively, that the way to circumvent jealousy, possession, or feeling "left out", is to find one person to share equally.
The rationale is that one partner cannot get jealous if he or she gets to do all the same things at the same time with the new partner as the existing partner. This does not refer to triads that just happen to spring up because the relationship between all involved was most compatible in a triad relationship. This is specifically for existing dyads seeking the Hot Bi Babe to "complete" their family and perhaps solve any underlying issues along the way.
That skeevy couple over there hitting on all the young females trying to find a Hot Bi Babe for their threesome is just Percivalian