Probably I was not clear- I am not going to restore any backup from 2016 to 2014. Question is about creating cluster with instances with different versions, i.e. adding to existing 2016 instance new one- 2014 as same cluster resource. There was post while ago on SSC regarding 2005/2008 combination on the same cluster and answer was- it's possible. I just wondering if it's still true for 2016/2014 instances? Unfortunately cannot find any MS docs on that matter.
So, you are adding a third node that will have instances of both 2016 and 2014. The existing nodes will also have 2016 and 2014 as well. If that is the case, I do believe that would work as long as you have the same versions of SQL Server supporting each other.