My aluminum wireless keyboard had only two batteries in it. The alleged "third battery" was actually not a battery, although, like the previous post said, it looks very similar. Meanwhile, after struggling and banging on the battery tube, I noticed there's a little diagram on the outside of the tube with only two batteries shown. So I followed the diagram, inserted fresh AA batteries, turned it on, and it works.
I guess some keyboards have three batteries, but mine had only two.
BklynPhotog