I understand how a general single input PID controller works, but I'm having trouble understanding how to calculate the ERROR in a multi-input PID controller? For the sake of clarify, let's take the example of a PID controller in a line-following robot. For a single light sensor line-following robot, the pseudo code is something like this:

Kp = 10 Offset = 45 MaxMotorSpeed = 50 Ki = 0 Kd = 0 LastError = 0; Loop forever LightValue = read light sensor ERROR = LightValue - Offset integral = integral + ERRORderivative = ERROR - LastErrorTurn = Kp*error + Ki*integral +Kd*derivativeEnd loop forever

However, how would you set up the ERROR variable for the PID controller of a line-following robot that has, say, 3 light sensors intended to go across the line in a straight configuration?

Any guidance here would be much appreciated. Thanks in advance!