The M3 Freightliner maintenance is scheduled for 24months/24k miles. Depending on the equipment on your chassis, it may include:
Engine oil/filter
Chassis Lube
Replace Fuel filter(s)
Replace air cleaner
Drain/refill cooling system
Change rear axle gear lube
Check electrical connections
Check ride height
Torque front axle spring bolts
Inspect drive belts
Inspect exhaust system
Drain air tanks
Replace air dryer dessicant cartridge
Inspect fuel tank, mount, lines
Visual chassis inspection
Check all fluid levels
Torque wheel lugs
Set tire air pressures
Transmission service
These items are directly from Freightliner's M3 service sheet. Note that all of the above items do not necessarily get done at this interval, depending on your equipment. For example, if you have 100% TransSynd transmission fluid, the change interval is 48 months for the fluid and 36 months for the filters. The air system filters change intervals have changed from 24 months to 36 months depending on what came on your chassis.
The list goes on, but note that the M3 is the major service point for the Freightliner chassis.