I have set up a scheduled backup under Configuration > System > Scheduled Backup Settings.
However, I don't know why but the system seems to be doing three backups at the same time.
This is causing a strain on the server.
Is there any way to stop them all and start afresh?
yes, use proper backup scripts. forget about magento backup.
You can use this free script to back up your store https://blog.amasty.com/how-to-backup-a-magento-store-tutorial-script/
Also, check out this extension https://amasty.com/magento-cloud-backup.html , it may be helpful