12c Automate the opening of pluggable databases after Container DataBase (CDB) starts up

These steps are documented in on My Oracle Support.

How To Automate The Opening Of Pluggable Databases After The CDB Starts Up? (Doc ID 1585529.1)

Create a database startup trigger to open all pluggable databases.

sqlplus / as sysdba
create or replace trigger open_all_pdbs
 after startup on database
begin
 execute immediate 'alter pluggable database all open';
end;
/

Or to selectively startup pluggable databases, use separate commands for each.

create or replace trigger open_all_pdbs
 after startup on database
begin
 execute immediate 'alter pluggable database MYCDB1 open';
 execute immediate 'alter pluggable database MYCDB2 open';
end;
/