Startup Pluggable Database Automatically after Container Db is Restarted.

 
Login as sys user & create below Trigger under CDB, to open all pluggable db's.

CREATE or REPLACE trigger OPEN_PLUG_DB
   after startup 
   on  database 
BEGIN 
   execute immediate 'alter pluggable database all open'; 
END OPEN_PLUG_DB;
/

If you want specific pluggable db's to be opened - specify them individually 

CREATE or REPLACE trigger OPEN_PLUG_DB
   after startup 
   on  database 
BEGIN 
   execute immediate 'alter pluggable database pdb1 open'; 
   execute immediate 'alter pluggable database pdb2 open'; 
END OPEN_PLUG_DB;
/