I would say that if we look at the history of North American defence relations and how they developed, you initially begin with a bilateral approach. So when we look at the Second World War in the 1940s and the early 1950s, the way that Canada and the United States began their cooperation was primarily bilateral to start with.
You really need this in order to understand developed common doctrines, common understandings, and common points of communications between the countries before you can really jump into something that's either binational, let alone tri-national.