I am a little confused. On pages 56 and 57 in the CWNA Study Guide Revision 3, it is multiplying the mW values by 3 when adding +3dB.
As the rule of 10s and 3s, it states that you should multiply by 2 when adding 3dB.
Can someone clear this up for me?
This is a typo in the book. The examples have been corrected in the errata document, which should be posted to the web site soon, if it hasn't already.
You are correct that a 3 dB gain should correspond to a doubling of signal strength.