Doom was “real” 3D in the sense that it displayed proper 3D environments with textures and all. You could go forward and backwards, left and right, and up and down of stairs/elevators. That’s all 3 dimensions.
The player might have technically been confined to a 2D plane, but I would say it’s 3D graphics.
But games with proper 3D movement came much earlier. One example is Elite from 1984, which allowed space flight with full 6 degrees of freedom.
Another example earlier than Quake worthy a mention is Descent from 1995.