When asked why support has been withdrawn for the Pixel 3, a Google spokesperson had the following to say:
We find that three years of security and OS updates still provides users with a great experience for their device.
The difference between past Google phones and the Pixel 6 duo is that the latter is powered by the company’s own Tensor chip, which has likely made it easier for the company to support the phones for longer. The use of third-party chips somewhat limits a company’s ability to provide software support, but that’s not to say it can’t be done, as demonstrated by Samsung, which has extended security support to four years and this even applies to some Snapdragon-powered handsets that were released in 2019.