I like to have backups for my setups, so I'd go with the sump and 1 fx5 or if you already have the 405's, them and the sump.use the canisters mainly for particulates and the sump for bio.
if you take the money ($500) that you would have spent on the other 2 fx5's and put it into a sump you can have a superior bio-filter for the same price. especially if you get as good of a deal on a 55G or 75G as you did on this one, and spend the rest on bio-media.