Mixed Editions in a 3 Node SQL Server Always on Availability Group
You might missed that the Availability Groups are only available in Enterprise Edition. 

In you scenario (with mixed edition), you would neither able to do following AG Configuration:

Availability Groups
Required all replicas running Enterprise Edition  

Basic Availability groups (limitations)
It limit to two replicas only  

Distributed Availability groups
Standard Edition only support Basic Availability Groups and Basic Availability Groups cannot be part of Distributed AGs