Replicate data for Reporting needs(not the entire database couple of tables from different databases). Secondary Sync node is for offloading the maintenance.
Here are a couple things.
Assuming you are going to have a 4th server to contain the replicated data, your licensing would be higher.
If this is the scenario, Primary, Secondary one, secondary two, and subscriber, then you need to fully license the primary and subscriber, and only pay for SA on the 2 secondaries.
If you make one of the secondaries read-only, and use it for reporting, you would save on licensing. That would be 2 full licenses and one SA, and opposed to 2 full and 2 SA. You would need to configure read-only routing so that whatever server in the AG that is secondary gets the reporting traffic. You do lose the ability to tune the secondary specifically for reporting however.
Also, when you say "offload your maintenance", have you considered the ramifications of that? Is this just backups, or is it backups, integrity checks, and other things?
You need to perform integrity checks on all of the nodes in the AG. Corruption can occur on any one of them. We do not perform backups on the secondaries, we do them on the primary only.