3 dB gain?
Or do you mean the frequency with a magnitude which is 3dB down with respect to the maximum?
ya generally when we consider enhancement of signal or even in control system a frequency with magnitude of +3 dB is considered.why do we do that?
No - not a magnitude of +3 dB.
I suppose you speak about a gain magnitude which is 3 dB LESS than the maximum value.
This corresponds to the definition of a corner (or cut-off) frequency. But note that this definition is completely arbitrary. The reason is (most probably) as follows:
A first order circuit has a maximum possible phase shift of 90 deg - and at the cut-off it has a phase shift of 45 deg. More than that, the POWER at this frequency is 6dB down which means 50%.
Thus, it is a frequency that has some ordinary properties.
As an additional comment:
I think, the 3dB frequency is particular important for amplifiers and filters.
In control loops, the time domain behaviour is more relevant (settling time, overshoot, step response, damping).
A signal with power less than 50% of original(maximum) is of not much use. Taking dB we get, 10log((P/2)/P) = 10log(0.5) ~= -3 dB. Hence in the dB scale the half power occurs at -3dB. The answer to 'why 3dB?' is related to intolerance to signal reduction by 50%. Had it been 25%, it would have been -1.25dB.
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