Trial and error not. Just simplifying it to make sure it works in that configuration, then adding complexity, a step at a time, to determine when it stops working correctly. What was added to it that causes improper operation is the cause.
Try each multiswitch with all three receivers -
I re-read my previous post and the second part should have read
If that doesn't find the fault:
Add the diseqc switches, one at a time, and run the other receivers directly to the multiswitches
Think it should 'hickup' when the bad switch is then connected. May be a particular port on one of the diseqc switches.