3 abreast or any combination of 1 x fat boat and 1 x nb or 2 x fat boat generally bad idea. Try and tie direct to the land not to the next door boat so they are not trapped. Never moor anywhere you will cause an obstruction to the navigation. Think sight lines as well as actual width sticking out into the cut. I'm not going to go on about lock landings, tunnel waiting areas and service areas as in London they've had to paint the bollards yellow and blue respectively to prevent the common sense challenged from using them for 14 day moorings
Check out http://canalrivertrust.org.uk/boating-in-london/london-boating-bulletin for more handy hints and news and also the London Boaters FB page.
Oh, always worth knocking before breasting up, you never know if you've just moored against someone you don't want to moor against for all sorts of reasons starting with anti-social behaviour, rabid dogs, super loud gennies, unsilenced engine, someone who believes it's ok to burn their rubbish in their stove or the local rave boat with monster speakers and they are all zombies or vampires who come out only at night.
Ok, I'll get my hat and coat now and run out to get some popcorn,
;-)
D