Why so many tanks and how to clean them?

[Syco67]

My sisters 69 GMC 3/4 ton 2wd has 3 gas tanks on it, was that a factory option, or an aftermarket deal? There is a selector valve on the floor between the driver door and the seat. The PO said he only ever used the middle tank because he was unsure of the condition of the other 2. How should we go about checking those tanks out, put in some gas, some cleaner, and see what happens? I know they are empty, I opend the valve, and nothing came out, not even a trickle.