Have a craftsman 19.5 H I/C Platinum engine. Sat outside for 3 years. Tried to bench start it with a new 12volt battery (engine is not in the tractor). Engine turns over one slow revolution at a time.

Cleaned all: contacts, carb, plugs, air filter, fuel filter. Won't start.

What can the problem be ?

Check the oil level to be sure its not over filled. If it is over filled check to see if there is gas in with the oil. A sticky carb needle will allow gas to drip into the cylinder over time over filling the crank.

Hi,
I am betting you have An Intek and it requires a vale adjustment somewhere in the first 200 hours of service. The valves should be adjusted as in the YouTube video:
Lawn Mower Repair Valve Adjustment - YouTube
Be sure to check the movement after adjustment of both rocker arms is equal.
Peace,
Clarke