That's not what I remember, Ultima III had crude 3D dungeons similar to Wizardry's dungeons. The maps of the dungeon levels were 2D and tiled. Google image search for "ultima iii dungeon" confirms my memory.
Wizardry's wireframe dungeon renderer was a bit more advanced, though, in the sense that it supported empty space and disjoint 'islands' in the map, while Ultima I and II could only display corridors.
IIRC that limitation was fixed by III, but it may have been IV. It was definitely fixed by V.
QotA is really awesome, I should play it again some time.
Of course, in Ultima III, you couldn't leave the top-down combat arena until the monsters were dead or you were. That was also fixed in Ultima IV, where you could chicken out at the cost of some Valor.