Defenders always seem to make a beeline for point B, maybe because it’s closer to their spawn, maybe because they believe it’s easier defend and hence harder to die, or perhaps because they’re simply in the habit of doing so. I certainly think B is easier to defend once the defences are set up, which is why the attackers often look to start with B, to take it out before it can be too heavily garrisoned. However it often happens that the attackers, expecting heavy resistance at B, will first attack and capture A for the time bonus, and to get it out of the way. They usually face very little resistance.
I once again insert the not so subtle paragraph concerning teleporters. As a defensive team you need at the very least 2 engineers. While you can simply give up on one point and focus on the other, I wouldn’t recommend it. Even if you can only hold out defence at A for a minute, it’s better than nothing, and assuming they don’t take B in the meantime you lose nothing. Therefore I like to see 2 – 4 teleporter entrances outside spawn, about half and half going to A and B. Even if the enemy has half their team in A, sometimes all it takes is a pyro to rush in and cause panic while a demoman fires in some sticky bombs from below to remove them all.
However once a lot of defenders are in a point together, it is generally very hard to remove them, so your objective should be not so much stop them from capturing the point but stop them from getting too many men into the point in the first place. This is why I suggest planting 3 or 4 men at A (including a medic) so in the case that the attackers decide to rush A, the handful of defenders can at least hold them up until backup arrives, either through C or through the tunnel (although through C is better because you come out on higher ground). And defence should definitely be scrambled to A if no one appears to be attacking B.
So in summary for the first two points, get an engineer to get set up at A (teleporter exit included) along with a medic and a couple of other guys (soldiers, heavies, pyros and demomen do well). Just the sentry gun and a bit of harassment to prevent the attackers from targeting it can be enough to hold them off A for the whole game, assuming they don’t make a real effort for it (i.e. they’re split between A and B). To defend B, get a soldier or demoman on the roof (see tips), get at least 2 or 3 sentry guns set up (see vantage points), maybe a sniper watching the long exit and a heavy, pyro or demoman watching the short exit. Ensure a medic or two are running around checking people’s health.
Defending C should be easy but half the time it seems not to be. A few simple steps should be taken in the event of point C coming into play. Until the defence is organised, an engineer should get up the top of the tower and build a teleporter exit and dispenser, a soldier, demoman, or especially heavy should get up on the top floor and spam any attackers they see, and the other 3 or so engineers should be getting their sentry guns set up in various positions again (see vantage points). Having some medic and soldier/heavy combinations patrolling exits should also be quite a deterrent. Finally, a freelance spy should be running around behind enemy lines sapping teleporters any chance he gets. Since point C is just outside defensive spawn, you don’t want to lose that transit advantage by allowing the enemy to teleport in.
