Game concept
Trilateral Interaction
The game also links it as Trilateral Interactions.
From the game's encyclopedia
Trilateral Interactions require not only an approval of a recipient, but also of a third party.
An example would be Borrowed Powers, where a Liege may use their Diarchy Veto.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Trilateral Interaction is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Trilateral Interactions require not only an approval of a recipient, but also of a third party." It is part of Interaction. The game also links it as Trilateral Interactions. Patch 1.20.1.2, from game/common/game_concepts/04_ep2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/04_ep2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.