In RC LPF, what is the magnitude at the pole location?
So As per the pole concept the magnitude response at this pole should
  be infinity. But why it is -3dB?


You are thinking that the bode plot will indicate the position of a pole. For a simple RC filter the pole does not coincide with the jw axis therefore it cannot reach infinity on the bode plot.

Here is how a pole might be seen relative to the bode plot: -



The diagram is taken from a 2nd order system (hence there is some peaking in the bode plot) but the principle remains - the bode plot may or may not show a peak and this implies that the "infinite" pole "lies behind" the bode plot and the effect of that pole diminishes the peak seen in the bode plot as it moves further away.