Change build cron from every hour to every 3 hours

There is no need to check so often even it doesn't make any load

.github/workflows/cron-beta-repository.yaml

@@ -3,7 +3,7 @@ on:
   push:
     branches: master
   schedule:
-    - cron:  '0 * * * *' 
+    - cron:  '0 */3 * * *'
 jobs:
   build: 
     runs-on: [runner2]