@Maxine Stockton this Trellinator function will loop through all cards in the board "My Board" and add 3 new items to the checklist "Target Checklist Name":
https://gist.github.com/iaindooley/6b414089d8d421eabdd7b59762e03bf7
You can read more about Trellinator here:
https://community.atlassian.com/t5/Marketplace-Apps-articles/Introducing-Trellinator-Automate-Trello-with-Google-Apps-Script/ba-p/925271
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