If you have at least 4 sockets to cover both nodes of the small HA cluster, you can simply revoke the license from the existing nodes in Veeam console and they will be assigned to the new cluster during the first run of the backups or replica.
About the third node, it doesn't matter in terms of licensing if it's a replica target or a backup target, as Veeam licensing only counts source sockets and not target sockets.
I hope anyway that replica will not be your only protection method, and you will also have proper backups.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1