I have a AG with 2 nodes in a site in sync mode and 1 offsite in async mode. I'm having trouble understanding how the logspace is maintained in this scenario. For instance, I'm taking log backups every 15 mins on the primary server and on the secondary server in sync modem. If I move my log backups off to the secondary node (in sync mode) does this mean it will release space on the primary node too, or do I still need to run tran backups on the primary node, which will in turn release space on the secondary nodes too?

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When you run the log backups from a secondary server, when the log backup is complete the secondary replica will tell the primary replica that the log backup has been completed and what the last LSN to be backed up was. The primary replica will then mark the VLFs which were backed up as inactive. That change (which is logged) will then be transferred to the secondary replicas.

In other words, no you don't need to take backups from the primary replica as well as the secondary.

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