What is an "IR" ? Is it an IR distance sensor? If so, what range does it have?
What are the three servos? To drive the turret, or to drive the sensor positions?
In general, what you want to do is have the three sensors point in a 30 degree fan: slight left, front, and slight right.
Then, check which sensor has the nearest distance-to-something. Turn the turret in that direction, if any. If it's straight ahead, fire!
Use a cut-off of readings beyond which you don't do anything.
Also, if you don't have any detection, you might want to scan the turrent left and right to see if you can pick something up.
Finally: IR distance sensors just pick up "what is the distance to <whatever> in front of me" which may or may not map to a person. It may map to a pillar, or an infant stroller, or a dog, or whatever.