I had a lot of vibration after changing the prop  to a 3 blade MT on my
360. The vibration was pretty bad, almost as though the engine was
missing. Most were saying I needed to balance. A wise mechanic changed
the engine vibration isolators and the vibration disappeared.

Don,

 

Interesting.  A friend with a 6-cyl Bonanza went to a 3-blade prop and could not get rid of the vibration.  A graybeard noted that bearing wear after many hours on a two blade prop is different than that on a 3-blade.  Go figure.