I would recommend replication for the Dupes and Log Shipping for the Main Backup. This is under the assumption that the Main Backup server is intended to be a fail-over to the Primary in the event of a unlikely disaster situation.

Implementing replication would be fairly quick and easy. You could create the publications on your Primary and create multiple subscriptions to that publication on your Dupe servers. It would be a good solution for keep all of your data in sync.

If your Primary is a "busy" server you'd probably have to schedule a maintenance window to generate the initial replication snapshots...but after that you'd be good to go