i just tried to ring you but your out, lol.
if the filters supplied are prefilters then you don't need them with a merlin, as the top chamber of the 3 is a prefilter, but if you have it use it as the 10" ones are much cheaper than the merlin ones to replace.
if one of them is indeed a Di filter, then connect this to the pure water pipe to polish the water afterwards.
i don't have a pic set up like this as its not the same setup we use.
Justin.