You may have to use a plugin or a synchronous workflow assembly. Then just pull the other email addresses based on the recipient in the TO field, add the other 2 email addresses to the Activity Party for the CC field.
If you do the workflow assembly option, choose create record and set the record type to email. This will create the email activity in a pending status which lets you open and edit it before clicking on send. (choosing the send email option
will create and send the activity without letting you make changes.) Then you will be able to confirm that the email addresses are populating into the address field.
Either way, you just need to have the custom code add the email addresses as the record is created and before it is sent.
Jason Peterson