SSRS HA Deployment Advice
You have to take in count that a 3rd box with SSRS counts more license.
In that case you have to place the SSRS databases on the alwayson cluster and the SSRS website on the 3rd box. You will not have HA solution for it and you will suffer downtime for server maintenance.
Otherways, if you choose the scale out deployment on the alwayson cluster with a net balancer in front of it you will have redundancy and no increase in license costs.
But you need a new component in the architecture, a network balancer. You can't enable microsoft NLB on the clusters' nodes because it is not compatible with the cluster itself.