Well, if ya get it for free--here's what to do:
(Remember, if it was completely submerged for 3 years, it might not be too bad inside the engine, etc. Seals keep the oil in and water out of the tranny.
Only thing you can do is first--don't pay any money for it! Then if you know anything about engines, take the engine out and take it apart, and look inside the lower end. If it appears very rusty, then use it for a boat anchor. Check the rest of it the same way. But i'd say, with that long under water, you'd might as well consider it as a lost cause. The transmission will probably be gone, too, so that leaves a bunch of rusty parts that are not worth much!
If it had been fished out soon after it fell into the water, it could have been saved, probably! I know a lady who fell into 4 foot of lake water while mowing with her Wheel Horse tractor. They fished her and the tractor out, got everything dried off, including her, and both are still working!

Junk yard, I was scouting some MG parts,,, guy kidding with me asked how much I'd give him for an MG sitting submurged in his pond. Told him I'd take it off his hands for ten bucks. Held out my hand. chuckled..

Anyhow his driver showed up with one I had paid for, and the one still dripping from the fresh pond pull, handed me ten bucks.

It's motor wasnt bad. I expected lots of crud and rust but you'd hardly know it had been an anchor. Long time ago, Probably boil, polish journals, hone... about it.
I'm sure the transmission was fine too,, water isnt going to hurt them much, just bearings/bushings/shims, maybe some springs,, minor stuff.

Curious if anything every happened. I have a s370d and maybe able to use it for parts in the furture.

A little different story here. My BIL gave me a 1991 Craftsman GT6000 that had been completely submerged in a flood for several hours. It sat at his house for a year before I got it. I had to pull the heads because the valves were sticking (a little WD40 cure), changed the oil, cleaned the carburetor 3 times, drained and cleaned the fuel tank, greased everything real good, and I still use it today. He gave it to me in 1996 BTW.

Dave

I am curious if you opted to fix the tractor or is it still under consideration?

I am not sure with the engine but the rest is take it all apart I have one that the tran was full of water the clutch was rusted to the press plate. I had to take the clutch out to get it working free and take clean all the lines out. it is a lot of work