Is there an advantage to having a rectangular aquarium over a square?
The biggest advantage to rectangular tanks is lighting. But maintenance is also often a factor. Kind of difficult to reach 3' into the bottom of the tank to work on it.
How much lighting would I need with this depth?
For 3' of water you'll need a considerable amount, especialy if you plan on having any corals, clams, anenomes, etc. Quite likely a number of 400w MH
Is it possible to be able to view all 4 sides without any unsightly piping or anything?
Yes. You'll have to put plumbing up through the bottom of the tank to provide for the various inflow and outflows. This plumbing can then be hidden by live rock etc.
I need filtration suggestions as well, what would be an optimal set-up?
Optimal would be a decent sized Sump and refugium located elsewhere, either below on a floor lower or in a room next to the aquarium. The majority of filtration would be done by the LR
and the fuge.