The third circuit shows a Mixer with input attenuation. This is a fairly simple concept. A potentiometer is placed in the signal's path between the source and the summing resistor. When the wiper of the POT is at the top it simply represents a 10K load to the source. 10K is a pretty high value and most line level devices can easily drive this load. With the wiper at the other end of the pot it still represents a 10K load to the source but the input is effectively at ground (Shorted out) so no signal gets through. With the wiper in mid way position the input loading is still 10K, however the signal has to flow through a 5K resistance and is also dumped to ground by 5K. Halving the potential reaching the input resistor/summing node.
Well hopefully I've provided enough information so you could go out and roll your own designs. And hopefully I've been able to work it in such a way that it's relatively understandable. If there are any mistakes, errors or omissions, please feel free to point them out. But Please no nit-picking. I'm only doing this because of the number of questions asked on this subject and the relative interest for people to design their own.