The bundle includes 19 extra multiplayer maps tallying up to 38 in total. It's coming out February 15th for $29.99. Two map packs are free over Xbox Live.
So the triple pack comes with Gears of War, Gears of War 2 and the 'Road to Ruin' campaign add-on. Then there's all those extra maps spread across the two games. It doesn't have on disc GOW's Map Pack 1 or the Hidden Fronts Map Pack.
However those two can be downloaded freely from Xbox Live. No the Triple Pack does not include beta access to Gears of War 3, unlike Epic's soon-to-release Bulletstorm.