There are three- two ambers and a red. Maybe I've never paid close enough attention, but I didn't think mine illuminated to indicate which side was detecting an object. It's an amber light on one end of the three and slow beeping at first. Then, as I get closer to an object, there's a middle amber and more frequent beeping. Then, when I'm about a foot from an object, the third light, which is red, illuminates and is accompanied by a solid beeping.
I've had mine beep once and illuminate an amber light when I first put it into reverse and the light stayed the same until I took it out of reverse no matter how close to the garage wall I got. This was resolved immediately after wiping off the sensors.
If yours are clean, then I'd say you have a failed sensor. If there is no separate detection for the left or right side, then you can't simply swap sensors around until the problem follows. I doubt you can just unplug one sensor at a time in hopes the system ignores the faulty one but operates on the remaining ones.
I'd diagnose it just as you mentioned- by replacing one position at a time until it works.
