BigTuna:
How would DIR concepts apply to a 3-person team? In particular, what should be the procedure if one of the 3 is OOA? What about other emergencies, like snared, toxing, etc? How to decide who does what for whom?
With the qualification that a lot of what I am about to describe requires a GUE answer more than a DIR answer since the guys that actually teach this stuff sit around thinking about this from a new diver perspective and I don't, let me add a bit here.
In theory, the more people you have in the water to watch you, help provide solutions, carry back up gear, the better. In practicality, most dive teams fall apart, communication becomes a problem, you can't watch everyone effectively, etc. after you go above three people. The same thought process that goes into the reasons for three lights, not 4, one depth guage, not two, etc. applies here. As a historical aside, three man teams (not three people on a dive) developed out of the cave community and the limitations on adding more than 3 members to a team is even more obvious in that environment.
While some of the other answers describe a few examples of how you would actually see a three man team work, they miss the wider view that a dive team is far more fluid than that. At the start of a dive everyone may be assigned tasks. As a dive develops, things come up and until a group of divers have worked together for a while, it can be difficult to seemlessly address these issues. In DIR, the standardized responses help make the transition to using three man teams easier. However, for newer divers, the increased task loading of keeping up with an extra teammate and reacting quickly to situations without getting in each others way makes a two man team a better solution for most of their diving. That is not necessarily a DIR answer.
When I do cave exploration dives I am usually the weakest member of the team. However, these are very experienced teams and tasks are split and miscommunication is minimal and of a level that very few divers would even consider a failure in communication in that it usually only costs the team seconds. On these types of dives, the diver limiting what can be accomplished generally is very vocal and honest about the limits of the dive. (Not always the case, egos do get in the way.)
On the other side of the spectrum, I am generally the most experienced diver when doing open water reef divers. Here, when the team is composed of one or more divers without significant experience there is often less willingness/ ability for the weakest diver to define their limits. There is a greater tendency to act less like a team and for one or both of the other two divers to become dependent on the stronger diver and look to them for all of the problem solving even though they should be capable of handling any problem that comes up on a dive within their limits. While these problems can be addressed with detailed pre-dive discussions, it is easier to develop a true teammate with two person teams initially. Once again, not necessarily a DIR answer.