The 8-pin IDC connector going to the Desire XTI from the termination board is missing. Can I use a 3-pin connector instead?
No. The DMX line goes out to the fixture on pins 2+3 and through back to the termination board on pins 5+6, with pin 1 acting as the common. Even if the fixture is the last in line and you do not need to run a thru line out to another fixture, the termination switch will not perform its intended function if pins 5+6 are not connected and it is not good practice to solder a 120Ω resistor across CatV wires. However, because only pins 1-3 are used on the DMX In and Thru connectors on the termination board, it is OK to use a 3-pin connector there if you lose one of the 8-pins.