My real problem isn't the previous 3 year window but that it counts from the first day they sell it, not the last. I bought my current phone, a Pixel 3a, late in its cycle for cheaper, early in 2020. It's now basically at the end of its updates because the guarantee counts 3 years starting from the release date in mid-2019, not from when I bought it.
5 years is obviously preferable but I'd like them to have also shifted it to be based on when they stop selling them new.