steps to create 12c cascade standby database , got errors to add the second standby into dg broker

Here is the setting information:

Oracle VM, standalone primary db PRIM, Standby1: STBY1, and we created two standby in a different data center called STBY2, STBY3.  HAS and db versions are 12c.

Now we want to configure STBY2, STBY3 as cascaded database, and STBY1 as cascading database.

So I have STBY2,3 instantiated from STBY1.

And want to add STBY2 and 3 into DGMGRL as 2nd,3rd physical standby database. Then we will do redoroutes to configure cascading.

However I could not add them into the dg broker.

DGMGRL> add database STBY2 as connect identifier is STBY2;

Error: ORA-16698: LOG_ARCHIVE_DEST_n parameter set for object to be added