Two active + one passive node for failover clustering
I'd like to know if it is possible to have a single passive node for two SQL Servers in a failover clustering deployment.


Yep that is quite possible. You can have 3 node(let us use node instead of server) cluster with each SQL Server instance on one node A and Node B respectively  and 3rd node is their in case of failover.

You can also have two instances running on same node(suppose Node A) and other node(Node B) acting as passive node. But such configuration is not a best one which you should go with.


  If it is apt to do so, will server C run two instances of SQL Server; one for A and one for B?


In case of failover, and both nodes A and B down, yes node c will act as owner of both instance running on Node A and Node B.


  As a side question; what is the benefit of "dynamic quorum" and "dynamic witness"?


You must understand witness helps in forming quorum. I would suggest you read this blog Understanding Behavior of Dynamic Witness. If you then have a specific question feel free to ask.