Dear Brother
This isn't a myth, no rocket science is involved here. The Electrician joins the electrical circuits of different portions of the house with the incoming three phases from wapda while the neutral is combined for all the three circuits involved. The point is that Electrical Load should be equally divided in to three portions (it may be between different floors or between different rooms).
The answer for your question that whether the 18A load on 3phase will be charged/billed same as 6A load on single phase meter is a big NO, the charges will be roughly be the same (including some other factors like meter rent, load connected etc etc). You will simply be charged for the net amount of Electrical Energy consumed.
Regards