If your studs are ~20" apart, 3 studs would be 40" apart. Since the unit is about 45" wide, you should be able to mount the unit on the first and third studs. There will be a slight overhang of the wire mesh at each end, but that is ok.
The length of the wire mesh is about 45". The uprights have to be mounted to studs that have a total spacing somewhat less than that. Some overhang of the wire shelf is expected at each end. Why this unit doesn't accommodate 48" spacing is a mystery to me.