It’s extremely intuitive, it’s part of the ‘us-west-2’ region, but an extension in ‘lax’ and it’s the first availability zone there ‘1a’.
Your suggestion makes no sense, it implies an entirely new region ‘us-west-3’ but with an entirely different naming scheme (no regions have an ‘a’ on the end of them) and the ‘local’ provides zero context about it with no expansion capability.
Wow, yet more obscure non intuitive naming from AWS. Why not just us-west-3a-local?