Gears of War 2 was definitely the most inconsistent thing ever, but this game I am consistently going positive and either placing first or second within the games overall scores and kills at the end of a match.
If you are not a seasoned veteran you will not being seeing consistency, because you are ill equipped in comparison to others who may have Gears 1 and 2 mastery over you. To add to this, there are even Gears 2 players who are struggling simply because they think all they need is a Gnasher 24/7 and are stuck in the mindset of swapping to it the second they spawn, pop a few shots, and then active it up once they are near a starting hot spot. I had a group of people calling the Gnasher shit in this game and saying they wanted to go play Gears of War 2 instead of 3. I laughed long and hard over that.
As much as EPIC has done for more casual players or newcomers to the series here with Gears of War 3, it isn't going to be enough to even up the new with the old. Older players can adapt far more quickly.
You'll just have to see what others are doing at the end of a game that got them to first place. Odds are they rushed a key power weapon on the map, got kills with all its ammo asap, and then returned for a fresh batch of ammo because the repop time for a power weapon is insanely fast here and feels like 30 seconds.
Learn map layouts, learn which power weapons cause more havoc in your hands, and try your best to use these tools while staying close to at least 1 team mate. You'll start seeing more results when you are the guy who always has the boom shot.