Running uname -m Gives x86_64 On M1 Mac Mini
I can reproduce what you are seeing on our Macs on 11.2.3 and 11.3.1 - we get arm64 / arm / arm64 reliably back despite different shells and homebrew (or lack thereof) setups on each.
bmike@mini ~ % sw_vers                 
ProductName:    macOS
ProductVersion: 11.3.1
BuildVersion:   20E241
bmike@mini ~ % /usr/bin/arch
arm64
bmike@mini ~ % /usr/bin/uname -p
arm
bmike@mini ~ % /usr/bin/uname -a
Darwin mini 20.4.0 Darwin Kernel Version 20.4.0: Thu Apr 22 21:46:41 PDT 2021; root:xnu-7195.101.2~1/RELEASE_ARM64_T8101 arm64

Choosing rosetta to run terminal changes this for a stock Mac. Quit and start again terminal.app, and I then get i386 / i386 / x86_64 the same as you reported.

https://www.jamf.com/jamf-nation/discussions/37723/rosetta-2-extension-attribute

If you look below, you'll see it enabled which will launch a shell that reports what you are seeing as opposed to the results &quot;out of the box&quot; that I showed above.