The evidence is pretty thin, but the twitter population seems convinced.
There are only two real pieces of evidence that have lead people to believe that Spark Unlimited is working on Star Wars: Battlefront 3. One, on its website Spark Unlimited lists in its job section that, "We are in production on an unannounced high-profile, popular game sequel for Xbox 360, PS3 and PC release." The other piece of evidence, is that it simply isn't flat out denying that Battlefront 3 is the title it is working on, on twitter. Every inquiry is met with a, "you'll have ask Lucas Arts about its franchises, we're just a developer, not publisher," response.
It's not much to go on, but it's possible.
Spark Unlimited only has a few games under its belt with Call of Duty: Finest Hour, Turning Point: Fall of Liberty, and Legendary. It seems more likely that they could be working on a new Call of Duty having experience in the franchise already, but twitter seems to think otherwise. We will know more next year when official announcements start rolling out.