While this is generally true, it's _a little_ less true the larger the model is.

Also, quantization techniques have improved - the I in IQ3 stands for imatrix - Importance Matrix - it is a bit more surgical in what it cuts. The result is a model where the most important weights are even Q6 or above, the least important Q2 or even below, overall it takes the space of a Q3 but with better results.

To add to the other comment, there's also Ridge quantisation - the majority of weights are indeed Q3_x, but the most sensitive layers are FP8.