After a bit more research, it looks like what I really need is a simple summing box?
I found a schematic for a passive 8 channel summing box here: http://www.forsselltech.com/8chsum_2.pdf
However I would need 3 channels at most, so I'm not sure how that affects the chosen resistor values if I modify the drawing? Does anyone know what the ideal values for these would be?
Ideally I'll be summing the outputs of a Buzz Elixir, LaChapell 583s, and an Avedis MA5. The summed output will then be fed into a buzz Essence ( or tonic) both of which have 20k ohm input impedances.