Several things can cause MMM vibrations with bent, damaged or knocked out of balance blades being the most common cause. Remove each blade,inspect and test balance. Testing balance is as easy as securing a large round philips screwdriver in a vice and putting the blade on it. If one side hangs lower take a grinder and remove alittle metal until it balances. Many L&G stores carry fancy bubble gauges so you can use one of them too.
Badly nicked blades should be replaced.
With blades balanced and reinstalled try the MMM out. If it still has a bad vibration, remove the belt so all the spindles and idlers turn freely. Spin each one, if one feels rougher than the others or makes any kind of unusual sounds you've got a bad bearing. There should be no side ways play in the spindles or idlers.
Replace/repair any that aren't nice and smooth or has side ways play.
These Kubota spindles and idlers are pretty stout and if kept properly lubed should last a long time. Course anything man made will fail eventually.
Lastly if the belt has any large nicks and/or is starting to shred, replace it.
Don't throw parts at the problem, troubleshoot and repair.
Good luck
Dave