Run GH Actions workflows for 3.x branch (#1842)

.github/workflows/build.yml

@@ -3,7 +3,7 @@ name: Build
 on:
 
   push:
-    branches: [ main ]
+    branches: [ main, 3.x ]
     paths-ignore:
     - 'doc/**'
     - '**.md'

.github/workflows/pr.yml

@@ -3,7 +3,7 @@ name: PR Check
 on:
 
   pull_request:
-    branches: [ main, release/2.x ]
+    branches: [ main, release/2.x, 3.x ]
 
 jobs: