Good on you for collecting them as they are often sold very cheap. I pick them up from op shops for $2~$5.

As to the 3rd wire,, well if it powers up and you have DC voltage at the other 2 wires then obviously you won't need to connect that 3rd wire. (others here might have more experience with those)
Some come with 2 wire mains input but some may have a 3 wire IEC Mains input socket. With those you may find the Mains Earth is connected to the Negative of the secondary wire, in which case you already have a ground return path to Mains Earth which in my experience has helped with interference.
Phil.