The -3dB point is the point where a signal has dropped to half it's peak power.
Assuming peak power=P
Half peak power = 0.5P
Convert to decibels, 10*log(0.5P/P)≈-3
db stands for decibel.In log scale we measure voltage,current or power.-3 db point is that where power reduced its maxinum value by halfor voltage or current reduced its maxinum value by 1/√2.
I would just add that usually when your talking "dB" you are dealing
with power (dBm). 1Watt=30dBm, 10W=40 dBm, 100W=50dBm and 0.1W=20dBm.
and if you go want half or doubble your power you need +/- 3dB.
P state for power,
-3db means power reduced by half,
3db means power doubled, and so no,,,
so you don't need to multiply of divide any more just add and submit to present power increment or decriment
Apart from all the above explanations, I would like to add that 3dB is the standard used to calculate the bandwidth of a particular signal i.e., 3db bandwidth is the bandwidth between frequencies where the power reduces to half of the peak power