Following are the standard cluster mode configuration.
1) Single node Cluster
2) 2 nodes (switched or switchless cluster)
3) Multiple Nodes in HA Pair ie. at present you can go upto 24 nodes (12 HA Pairs) for NAS and 8 nodes (4 HA Pairs) for SAN Environments.
You should ONLY add HA Pairs to an existing cluster which is greater than 2 nodes as it would help in Takeover and Giveback of the storage if one of the controller in the HA pair fails or goes down for some reason.
In your case, the existing 2 nodes will be in a HA pair (Generally). If node 1's (controller) fails the node 1's storage can be accessed via node 2 (Controller) via MPHA and vice versa.
If you add a singe 3rd node, in a situation where the node 3 (controller) goes down, the node 3's storage will not be accessable as it doesnot have a HA partner leading to outage.
You can do a (disruptive) head upgrade from the single node to one of the new systems configured as single node. Then, when the cluster is running on the new controller, add the second node (HA partner) to it to make it an HA pair.
Or, find a (temporary) second controller of the same model as the single node you have (you can borrow them through your Partner from NetApp, it's called Swing Gear), temporarily make a 2-node cluster out of your single system, and then use the regular way to upgrade your cluster hardware and remove the old controllers