Add Commit-Queue+3

This ensures CQ never shows up as a checkmark but always as "+1" or
"+2". Nobody has permission to vote CQ+3.

Change-Id: Iaaf6c5a47ac4db02986ccf01f8181fe94ee2822e
Reviewed-on: https://pigweed-review.googlesource.com/c/Public-Commit-Queue-Projects/+/151910
Reviewed-by: Taylor Cramer <cramertj@google.com>