There's no good reason I know of that the Q700 had only 2 NuBus slots. The case allows for three, and there's room on the mobo for it. The Q700 has a 150 Watt PSU, which I imagine would be enough to handle 3 cards.

In fact, one of my favorite 68k Macs I've ever owned was a Q700 into which I swapped a Quadra 800 board. In addition to a faster CPU and system bus (33MHz vs. 25MHz), and faster RAM (72-pin SIMMs vs. 30-pin SIMMs), the Q800 board has the third NuBus slot. All you have to do is cut an extra hole in the rear of the Q700 case, and you've got a super-compact Q800 (without an optical drive).

Best,

Matt