Yea, 3 discs isn’t much, especially since the space is cramped you’ll likely have to use 2.5" drives. I know - I’ve stuffed 3 drives in my SFF, no way to mount them so one is glued to the underside of the plastic rack. (Dell)
Then there’s power to consider - an SFF doesn’t have much of a power supply (mine maxes out at 120 watts, not sure how much of that is available on the 12v wires for sata, as it’s routed through the motherboard!). I had to buy a sata power splitter to have 3 drives.
But… My idle power is trivial (12 watts), and when it’s converting videos it peaks at 80 watts. Fan only runs when converting.