You have the bow and stern cleats that you can use to secure those locations. For the amidship section you can take advantage of the exposed square tube railing (like at or near the gates) by using one of a number of brackets available and designed for that purpose.
These are available at pretty much all the on-line marine supply stores (Overton's, iBoats, West Marine, Defender etc.) Some are listed as "pontoon" fender clips or brackets.
As an example, here's a link to the Overton's page under "boat fenders". It lists fenders and brackets in their selection:
There are a bunch of FunDeck owners in the forum, they may have other suggestions as well.