doubleup,
The only time you’d want to put the WR-ADDA analog option card in SLOT #3 is in the [INS] mode to use it as an insertion across the AUX sends, Buses and the mixers L/R Master otherwise it will replace or be the alternate inputs to analog inputs 9-16.
And unless you have some nifty outboard A to D’s you should consider using mic/line inputs 1-16 as much as possible. While the 20-bit AD/DAs on the mixers aux 3-6 and the WR-ADDA card are very good the 24-bit converters on the mixers inputs are slightly better.
The WR-ADDA 20-bit 8-channel A to D and D to A comes up on e-bay every now and then…you might consider buying and reselling a mixer just to get one.
Good luck,