Question:  I have a PTX-3 transmitter that I want to use with a Matrix +3 frame.  Can the PTX-3 audio input be supported via the M+3 MVX-A8 port?  Will I need to construct a special cable to make this work? 

Answer:  Yes, the PTX-3 IFB transmitter audio input can be connected directly to a Matrix +3 MVX-A8 port.  The Matrix +3 / Eclipse MVX port is typically setup to provide a balanced line level input and output at 0 dBu.  The PTX-3 audio input dip switches would need to be set to support a balanced line level audio input:
dip switch 1 = up
dip switch 2 = up
dip switch 3 = down
dip switch 4 = down
Wire up the cable as follows:
RJ-45 pin 4 (solid blue)  to pin 2 of 3-pin XLRM connector
RJ-45 pin 5 (blue-white) to pin 3 of 3-pin XLRM connector
leave floating pin 1 of the 3-pin XLRM connector