as long as there's time involved and the client doesn't want you do it ASAP, you can always meet the escort on your know to get a feeling.
Whether a client is paying you more or less is between the client and you. As long as the client pays him HIS fee that's all he should care.
The Legendary Dave used to charge a fee to set up a 3some or 4some with a client.
Back to your question, many clients have the fantasy of a 3some and providing it or not is your choice. You can always ask him to make a few suggestions but even if he picks a guy you don't like... it's sex for $ anyways and even if he's not your type he might look waaaay better than most clients.