Anything outside 70-130 is suspect in practice, as the tests aren't well calibrated beyond ±2σ.

I'm not sure what intellectual performance is even supposed to be implied by any given IQ score, given that the scores commonly used today are just a linear mapping from standard deviations above or below mean.

That said, on the low end at -3σ I would expect a serious deviation from a normal distribution just from the effect of dementia.