How to find lower 3dB and upper 3dB from PSPICE frequency response
First, a graph without labels is no graph at all.  Ditch whatever irresponsible software created it and move on.

Second, if the Y axis is in dB and the X axis in frequency or log frequency, then it is easy to find the "bandwidth" of the amplifier by inspection.  In this case we are assuming you have defined bandwidth as the frequency range over which the gain ranges by no more than 3 dB.  This is merely the high frequency limit minus the low frequency limit.  This figure is of little use, with the frequency range (like 20 Hz to 20 kHz for a good audio amp) generally being more useful.  Saying such a audio amp has 19.98 kHz bandwidth is rather less meaningful.

To find the upper and lower frequency limit of the range that is all within your 3 dB spec, find the highest point on the graph, draw a line at 3 dB below that, then look at where that line intersects the plot.  Those intersection points are the upper and lower frequencies of your passband.  To get bandwidth, subtract the two.