add v3 to CI

Author: mgravell

.github/workflows/CI.yml

@@ -3,7 +3,7 @@ name: CI
 on:
   pull_request:
   push:
-    branches: [ 'main' ]
+    branches: [ 'main', 'v3' ]
     paths:
       - '**'
       - '!/docs/*' # Don't run workflow when files are only in the /docs directory