110s guns were always belt fed. The 110G-4s with Schrage Muzak had 60-round MG/FF drums for the upward firing guns. The gunner was sitting right there, so he'd re-load them. They only came with 1 extra reload per gun (2 guns). That's not the nose guns.
As for the 110 being designed for 3 men... I'm not sure where you get that. I've never heard that. The idea I've read was the "Zorstorer" plane was always meant to have a pilot and a rear gunner (at the time they thought it would actually protect the aircraft).
EDIT: when they switched from a single MG17 to the twin setup ("Z"-something) they were belt fed. They had to be, because they were side by side and the magazines wouldn't fit.