The Massey Ferguson 3120 belongs to the 3100 series and is covered here by more than 350 parts, including lighting, electrics and front axle components.
More than 350 parts are matched to the Massey Ferguson 3120 on this page, from GRANIT and from HELLA, DANA Spicer, Cobo and GRAMMER. They run from a small PTO O-ring all the way to a complete air suspension seat.
The PTO clutch uses internal and external plates, and you will find both here along with adjusting plates and PTO O-rings. For the brakes there are brake discs.
If you are rebuilding the PTO clutch, replace the plates as a set and soak the new friction plates in clean transmission oil for a while before assembly. Plates fitted dry can judder or burn on first engagement. The same goes for oil-immersed brake discs, which should be replaced as a full set per side rather than one at a time.
Starter motors, bushes for the lift arm and rear plates round things off.
Play in the lift arm bushes is easy to spot. Lower the linkage, take hold of the lift arm and move it sideways. If it knocks noticeably, the bush is worn and the shaft will wear faster too. Replace the bushes on both sides together and grease them well after fitting. With the seat, start with the console. An air suspension seat such as the Maximo Professional needs power for its own compressor, so make sure there is a suitable supply in the cab before switching from a mechanical seat.
The type plate tells you whether the tractor really is a 3120. Models within the 3100 range look very similar, so do not go by appearance. For clutch plates and brake discs the old part is your best evidence. Count the teeth, measure the outside edge and look for the number on it. Hold that reference number up against the product information. On a starter motor, the number is on its own small plate on the housing.
If you also run a Massey Ferguson 3125, it has its own page of matched parts. For every other tractor from the make, go to Massey Ferguson parts, where each model has its own overview and you can jump straight to the machine you need.