Looks like the main difference is with the motor. I *think* it is the same pump on all 3 (don't go by the Home Depot pics - go to the DeWalt website)
48055 $1,159 features a cast iron, twin cylinder, two stage, oil lubricated pump, a
heavy duty 240 volt induction motor, integrated control panel and is shipped with synthetic oil.
Equipped with an integrated control panel with air flow regulator,
two quick connect air outlets, tank and tool pressure gauges
18055 $1,418 features a cast iron, twin cylinder, two stage, oil lubricated pump, a
heavy duty 240 volt induction Baldor motor, integrated magnetic starter and is shipped with synthetic oil
Equipped with a pre-wired and mounted magnetic starter, pressure gauge and on/off switch,
high flow side port bung to deliver maximum air flow
18075 $1,698 features a cast iron, twin cylinder, two stage, oil lubricated pump, a
Baldor low 1725RPM industrial grade 240 volt induction motor, integrated magnetic starter and is shipped with synthetic oil.
Equipped with a pre-wired and mounted magnetic starter, pressure gauge and on/off switch,
high flow side port to deliver maximum air flow
The cast iron two stage oil lubricated pump features a one-piece cast iron crankcase, thermally stable cast iron cylinder body, aluminum head and valve plate, automotive style bearings, durable Swedish stainless steel reed valves, an oil level sight glass, an easily accessible oil fill, and a 12 inch cast iron balanced flywheel
http://www.dewalt.com/en-us/products/gear-and-equipment/air-compressors