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.
Sentinel may well qualify on some definitions of 3D platform (1986): https://www.youtube.com/watch?v=9V_pgo3vgiI
Video of ALpha Waves (1990): https://www.youtube.com/watch?v=qwFPV855sI4