Good locations for sentry guns:
Defending A is done mainly from the bottom or the top. At the foot of the hill sentry guns can be set up to cover the spawn exits, but ensure they are out of sight of any attacker who is yet to drop onto the dirt.
Attackers can be prevented from reaching the top of the hill by a sentry gun covering the final ascent, and the control point can be further reinforced by a sentry gun in the corner inside the building.
To cause trouble for attackers climbing the hill, defenders can also place a sentry gun on the roof of the hut just in front of the control point building. Build it as close to the control point as possible.
At B, the attacking spawn exits can be covered by sentry guns behind the hut at the short exit or behind the ramp to B at the long exit. A gun just to the right of the long exit can also target attackers from both.
Sentry guns inside the building containing control point B can be very effective if well maintained, and while the little enclaves are good spots, sentry guns can go anywhere inside depending on where attackers are coming from.
Finally, C has very few places that are both effective and protected from rocket sniping, but two good ones are on both ramps from defensive spawn, placed so that just enough sentry gun is peeking over to fire at attackers on the metal walkways.