I understand the A3 takes the same winch as the A2 except it has a hydraulic motor instead of a driveshaft.
Here are pictures on a hydraulic conversion on an M35A2:
M35 For Sale. Deuces for Sale Turbo Diesel, Army Trucks, Logging, Heavy Trucks, Off-Road Big Trucks, Four Wheel Drive
Near the bottom of that page look for "Fred G."
He pulley-drove the pump off the engine.
Unfortunately, his truck was scrapped. I bought his winch, pump and valve from an ebay seller. I'm planning to drive the pump off the PTO (also an ebay purchase). Got the winch mounted but not the pump yet. I tested the winch with my tractor hydraulics and really like the infinitely variable speed and forward/reverse with one lever. Would be interested in any info on the A3 winch such as line speed, etc.
Does your truck have a hydraulic pump already?