Triggering a build only when the change is pushed to a particular branch

We have one private repository and 20+ branches. We need to trigger a build only when change is pushed to a particular branch say branch3. For that purpose we created Webhook and added a rule stating that if Pull request is approved and merged then the build will be triggered. but we have observed that, build is getting triggered on any pull request approval. Eg. If pull request is approved for branch2 then webhook is being triggered. We want to trigger a build only and only when there is change pushed to a particular branch.