I just got my 61A750 working again, after spending $28 with an online tech, and coming to the realization that either the print in the samsung service manual is wrong (which I don't think to be that case) or here is the way to identify which is which. All the following assumes you have the back off and are looking at the tv from the back.
On the DMD block diagram in the svc man it shows FAN 1, FAN 2, and FAN 3, and which plug on the DMD board each fan plugs into. The DMD board, which sits pretty much in the center of the tv, has a couple of heatsinks with a fan right in front, blowing on the heatsinks. On the upper right hand side of the board are 3 - 3pin fan connections. The top one is FAN 3, the middle is FAN 2, and the bottom is FAN 1. If you trace those wires, you'll find that FAN 1 is the fan in front of the DMD board heatsinks. FAN 2 is on the far right, and toward the front of the tv. FAN 1 and FAN 2 are the same fans and FAN 3 is a different fan. It's located in the left end of a plastic enclosure that has a heatsink inside.
I ordered from samsungparts the replacement FAN 3. It was the right one. Only problem, FAN 3 wasn't the one that was broke. They all spun so you couldn't tell by looking that there was a problem.
I contacted an online tech service and he guided me to eventually measure the voltage on the yellow wire with respect to the black wire, on all 3 fans. It should read near zero, if every things ok. I measured all 3 and FAN 2 had 3.3 VDC on the yellow wire, indicating a problem. The other 2 (fans 1&3) had .02 VDC (OK).
I concluded my session with him with a big question mark on my face. The error message said fan 3, but fan 2 had the problem. There are only two possibilities for this outcome: the service manual is wrong about the connector layout, or, more likely, the firmware has a bug that is saying fan 3 when it's really fan 2 that has the problem.
Fans 1 & 2 are the same, somewhat smaller than fan 3. I removed the two screws holding on the powersupply that sits at the far right side of the tv. You slide it toward you a 1/2" or so, and now you can lay it over to the right, and have complete access to the fan. Remove the two bottom screws and it comes out.
I ended up replacing fan 1 in my preliminary foray into my repair ordeal. I was mistaken about which fan was which. So I now have a fan 3 fan in place of the fan 1, and the original fan 1 is the replacement for fan 2 that went bad, though it still spun.
Anyhow, hope that clears up some confusion.
BTW, I really love this tv. I can't believe they quit making them so soon. I've had mine for over 3 years, and this is the first issue I've had with it. I'll take replacing a fan any day to throwing a good LCD tv, that has a bad 10 cent part in it, in the trash. Hope spare parts are available for a few more years.