If by standalone you are looking at something like a reporting system - using read-only replicas from an availability group - then you may want to setup your cluster with a quorum drive. No need to use a shared folder - use a disk quorum in that configuration.
You also want to make sure you patch the servers so you can set the node weight on the standalone to 0 so it won't have a vote in the quorum.
Essentially, you will have a 2-node FCI cluster with a replica node hosting your reporting instance. You don't want the reporting instance to affect the cluster quorum or cluster health.