Yes, there is going to be a 3rd book. If you check Patricia Briggs' website, she gives out information regarding all her books and an estimated release date.
The next book is the 5th of the Mercy Thompson series released this month, then she will be working on re-doing other books that she had started. Then she will go back to the 6th Mercy book, followed by the 3rd Alpha & Omega novel. She estimates that the 3rd book will be out around 2012.