The three sensors are to measure different levels in the tank (empty, 1/2, & full) and relay the readings to both heads and the main panel. I didn't know before "diving in" whether they were gunked up or defunct. I was pretty relieved that they operated properly once I got them cleaned up. Easy enough to test while they were out of the tank. I left them hooked up and turned on the head system then had the admiral check the panel as I moved the floats. If you remove the floats (they are held on by a small plastic C-clip) make sure you put them back on the right way up or they won't read properly. I think there is a small magnet in the float. Also the bottom/longest sensor for the "Empty" indicator goes on the opposite way then the other two. Read that on CSR somewhere and it was helpful. Flush the tank a few times before you tackle the job. Good luck.
Well, it took all season to get to this job, but I finally just did it. After cleaning them up, I got all 3 sensors to work. Thanks for the tip about the "empty" indicator needing to go on the opposite way. That was a big help (as I initially had it on backwards). Not a fun job by any means, and access is terrible. Having long skinny arms helped me, but it still wasn't easy to get them out.