My Am Diesel 136 has two sensors, one on either side of the head under the tank. Same locations as yours.
One is a temp gauge sensor; the other is an alarm sensor. Both are single-wire although I don't know which is which and (of course) the wiring color is the same...
I purchased a replacement temp gauge sensor because I thought that one didn't work. The one Brian at American Diesel sent me was a "floating ground" with two connectors. The existing one ended up working when I did the engine run test, so I never installed the new one or traced the wiring to see which was which. I still have it in the spares box.
I am guessing that the third one you have in the tank was something added by an owner along the way for some reason.
The only way I know of to figure out which does what would be to trace the wiring either manually or with a chirp sensor. If you have butt connectors, you could disco them one at a time to see which one takes out the temp gauge, leaving the other as the high-temp alarm - but mine have threaded terminals with nuts, so that's not really an option.
I will be replacing all that wiring while on the hard so I can label them.
EDIT:
Looking back at your pic, you have two different color wires - yellow and what looks like gray or white. I'd call Brian at Am Diesel and see if he has an educated guess on which is for what. My WAG is yellow for gauge and gray for alarm, but there's a 50% chance I'm wrong, LOL