The studio "can't read" all they need to from their past catalogue on Xbox 360. It's to do with Xbox Live data restrictions. Previous Gears titles will still unlock.
"Turns out that we can't read the Bulletstorm, Shadow Complex, and UT3 achievements for unlocks in Gears3 afterall," programmer Joe Graf. "Sorry about that all."
"All of the Gears games are in our publisher/dev family, so those we will reward with unlocks," he later added. All of Epic titles are classified differently from one another meaning that data can't be manipulated directly.
Gears of War 3 releases exclusively on Xbox 360 later this year.