ANSWER:

The best is to fast the 3 white days but you can choose any random 3 days of the lunar month as well but it would be less in reward. 

i usually fast Mondays and Thursdays, and white days (13,14,15) every month. Recently I read that the 3 fasts (white days), a person can choose any 3 days of the month to fast. Is this correct? If so, then is there any difference in reward between white days and any 3 days?