What we can't control is how many people show up. This year, it was looking like we'd have an odd number of people pretty much continually through the week. A late addition and a delayed arrival now mean that we will have an even number of folks throughout, which is nice, but we had ways of dealing with that problem. Here's how we planned to deal with an odd number of players...
1) Games for three. This is a pretty short list, as wargames go. The majority of possible games seem to lie in the card-driven game system, which is nice on the one hand as most of us really like this style of game. However, it does lead to a certain amount of samey-samey when you do it for an entire week, and we like variety. In particular, we'd planned to play Napoleonic Wars and Here I Stand with three, although Napoleonic Wars is now officially off of my play list and Here I Stand is uncertain. Friedrich is another casualty.
Wellington is another CDG that works well with three. You can play Sword of Rome and Successors with three, but my experience is that you really need four to get the full effect of these games. Kutuzov is also playable with three, but it doesn't have the natural division that Wellington has (with the Brits and Spaniards).
2) Monster Games. Any game that benefits from team play, which usually means very large (or "monster" games) can often work just as well with three players, one taking a single side. If the game is very large, you can even take a page from Kutuzov and have the players essentially play against themselves. For example, let's say that you're playing a game that involves the German invasion of Russia in 1941. The Germans divided their forces into three Army Groups; North, Center, and South. The Russians were similarly divided in terms of their forces, into military groups (although the rapid advance of the Germans forced the Russians to reorganize more or less constantly). If each player takes a German Army Group, and also takes a non-corresponding Russian Front, you can play the game as if you had six players, with the attendant rivalries and lack of cooperation.
We had not planned to use this option this year, but it would make for a very interesting monster game. You could do the same thing with Highway To The Reich, where someone took the 1st Airborne/US paratroopers/XXX Corps on one side, and the North/Central/South German forces on the other.
Obviously, this sort of thing requires players who are either more interested in seeing how history plays out than anything else, and people need to play each side as effectively as they possibly can. I suppose there's a good way to make this competitive without having your opponents try to screw you out of a win through collaboration.
3) Hotseating. Imagine a game of Paths of Glory where you play the Allies on Turn 1, sit out on Turn 2, then play the Central Powers on Turn 3. Lather, rinse, repeat. Like the monster game, there's a certain requirement to play honestly, but you could easily create victory conditions where whoever managed to take the most VP spaces during their turns won. On the downside, I could see how if you were in a position where that was unlikely you'd also be unlikely to try to make any progress, and in fact might just set your opponents up for failure by *giving* ground. Hopefully the fact that you're playing both sides would help balance that out.
Not only does this allow three players to play a two-player game, but it would be awesome for becoming familiar with a game, particularly useful for CDGs and the various combos. Downside is that you'd have some knowledge of what cards had been held over in a CDG. I have yet to try this system, but I could see it being very useful for teaching games.
4) Natural Divisions. Some military situations lend themselves well to a division of forces, especially in Western Europe during WW2 because of the combined operations of US and Commonwealth forces. Tunisia and Sicily (OCS), anything involving Market Garden or the full D-Day invasion, Cobra, Italy, the list is extensive. The same goes for ETO strategic level games, which often have a natural division of the Soviets and Western Allies and thus rules for playing with three players. However, a game like Breakout: Normandy is perfect for this, as the Allies have to bicker about who gets the impulses!
This will also work for games that group forces in some way. Flying Colors, a naval game, uses squadrons and commands that would work quite well with multiple players taking different groups of units. The Burning Blue could see multiple players taking over the various RAF commands against a single Luftwaffe player.
As it stands, the shifting realities of nano-con attendance, which once had roughly two days worth of gaming out of seven with an even number, now give us even numbers at worst for all but one day. I'd be a fool, however, to think that this won't be a constant "threat" in the future. I'm happy to realize that we have more options than the handful of games that are actually designed to be played by three - it's just a matter of thinking, sometimes literally, outside the box.