A stack of 2 switches forms a single logical switch.
So you first configure the stack, without considering the third one.
You now have 2 switches (one stack and one standalone)
Then you create a Ling Aggregation Group (LAG) between the stack and the third switch, with a link from each stack member to the standalone switch.
Note: HP use the term "trunk" for a LAG while usually this term refer to a link carrying several VLAN (and I profoundly hate them for that).
I am no ARUBA expert but from this documentation you configure the interfaces in LACP active mode with the command
switch(config)# int c1-c4 lacp active
The documentation is sparse about it though. Hope some other people can help you with more details about the config.