SQL Server AlwaysOn and FailoverCluster
A node can only be in one cluster at a time.  If Node3 is in CLUSTER-1, you have to remove it in order to add it to CLUSTER-2.

If you want to use Node3 with availability groups in both projects, then all five servers will need to be in the same cluster.

There are other options that would allow you to use Node3 for DR purposes without having all 5 nodes in one cluster.  For instance, you could perhaps use log shipping to replicate the databases on CLUSTER-2 to Node3.  More detail on the requirements would be needed for someone to be able to suggest what would be best.