Hi,
I decided to build my own aux switch with 3 switches and it's working great.
I used 3 3PDT momentary switches connected to one trs jack. One shorts tip to sleeve, one ring to sleeve and the third both tip & ring to sleeve.
If you prefer to buy one the first thing is that you need one with a balanced (stereo if you like) jack. Then, it have to do one of the three actions I mentioned before, it doesn't matter wich, since then you can program on TF settings what will happen when you push thebutton.
Hope I 've been clear.