Rock152 -
I think a lot of books do this. It's really just a question of making you an agent to keep your players separate and then counting the number of players each week. Some places will give you the house lines or allow you to place a man inside to move your own separate lines.
The costs to the book are in the $12-$20 range per player per week. This varies depending upon how much your business is phone vs internet. If you get quoted much over $20, keep shopping. You're not going to get that rate with 3 players, but the bigger you are, the more you can negotiate.
Don't forget, it's all profit for the book, as they are not taking any risks, but merely writing your action. If they charge $22, they're making at least 10% pure profit whether your players bet $5 or 5 dimes.