Yes, there is a third book in the "Found" series called "Revealed" by Margaret Peterson Haddix. This installment is the final book in the series and follows the characters as they unravel more mysteries and secrets surrounding their identities and the time-traveling adventures they embark on.
If you are talking about the books "found" and "sent" then the third book is already out. It's called "sabatoged." It's a good series, I have read it before, and I do recomend it.
Margaret Peterson Haddix's third book in the Missing Series will come out in July 2010. It is called Sabotaged. I think it will be about they get found
The Third book in the Ingo series is called 'The Deep'.
The third book in Cynthia Hand's Unearthly series is Boundless.
The third book in the Leviathan series is called Goliath.
The third book in Alyson Noel's Immortals series is called Shadowland.
The third book in the series is called A FAREWELL TO CHARMS
The third book of the Dormia series is called: The Shadow Tree.
The third book in the Resurrection of Magic series is titled "The Rise of Magic."
No, the third book in the series is called Spell Bound.
Chains of Reaction is the name of the third book in the Perfect Chemistry series.
The third book in the series is called Small Steps.