Just because Gears of War 3 brings an end to the story of Dom, Marcus and the planet of Sera's struggle against the Locust doesn't mean Gears of War is being laid to rest. Microsoft and Epic Games left themselves plenty of wiggle room in the press release announcing Gears of War 3.
"Developed by acclaimed studio Epic Games and available only on Xbox 360, 'Gears of War 3' plunges players into a harrowing tale of hope, survival, and brotherhood that will conclude the current story arc for 'Gears of War.' With the last human city destroyed and the remaining survivors stranded, time is running out for Marcus and his comrades as they fight to save the human race from the jaws of extinction."
The key phrase there is "current story arc." There's nothing stopping future Gears of War games. It's a similar position BioWare has taken with the Mass Effect franchise. Even though Mass Effect was pitched as a trilogy, the success of the series means Mass Effect could have life after the conclusion of Shepard's tale. Based on what we know, both franchises won't be dying anytime soon.
Gears of War 3 will be released on Xbox 360 on April 2011.