I think Sentinel wouldn't count as a "platformer" because you don't do any jumping (IIRC) - it's more of a tactical puzzle teleportation game. Not sure it'd count as "true 3D" either, really, not on the Spectrum anyway - it drew polygons onto a huge buffer you could pan around but the perspective never changed. But it was ludicrously impressive at the time, definitely.

[edit] 'aactical' isn't a word