First of all, fluvals have the greatest capacity for media, that what makes them so great, so why would you not want to use it to its fullest extent.
Carbon is not needed on a regular basis, It is only used to remove meds, or other chemicals. If left in for a long period of time. it can actualy have the reverse effect by leeching the stuff it pulled out back into the water. there is one other benifit to charcoal and that is to remove the tannins that bdriftwood can leech into the tank, other than that TOSS THE STUFF (well keep it for a med removal situation)
I woud fill the bottom basket with some mechanical filtration, sponge, filter floss, etc. to remove the big stuff ike food that gets sucked in, waste etc. the second tray i would fill with some even finer material to get smaller stuff that the first tray may have missed. and finally the third tray i would fill with some biological material.
The baskets in each tray allow you to be able to remove/clean part of your filtration instead of all of it at the same time. GREAT CONCEPT.
So again
Tray 1 (bottom) sponge
Tray 2 (middle) Filter floss
Tray 3 (top) Ceramics of some sort for your biological filtration
Hope that helps
Kevin