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MW3 was a shitty game. Don't feel bad that you felt that way. I also wanted to like it. But the game had some broken parts that made you think, "What the fuck?" There was just a lot of glitches. I still played the game regardless.
Destiny is a bore to me. Hey, that's just my prerogative, my opinion. I like sci-fi stuff, but the game felt ...bland. I wanted to progress a little further, but there just isn't something I wanted to do in Destiny. I've heard it's a loots a'fun, like Borderlands.
Black Ops 3 is a good game so far. Of the whole game, Single Player and Multiplayer are my favorite modes. Nightmares is nice, but it requires a lot of patience because you really have to finish a section to progress. Zombies is same ol' Black Ops Zombies. At some point, I'll spend some time to get far in that mode.
Anyhoo, back to your post. You just got to learn the maps all over again. But this time, with thrust jumps, wall walking, aiming and shooting, throwing bombs, setting up traps, etc. Find a class that you're comfortable with, and find a weapon that you like. For example: Because Kuda is your "starter" weapon, I progressed enough to get silencer, and start dominating. The thing is, the trick is to know where enemies are going to come from. Now, I'm not saying you should "choke point" locations, but really understand where you last were killed, and go back there and get your revenge. Sometimes they stay in one area, sometimes they don't. So, your job is really... to understand where the 'hotspot' is. It helps to have a reflex - which is what your friends were trying to tell you. Aim and shoot.
Some players don't really check their corners, so like I said - learn the maps, sneak into the hotspot(s) and kill 'em when they aren't looking.
How to kill folks in the air? Well, act fast. Remember: Reflexes. Sometimes you get 'em, sometimes you don't.
Practice, practice, and practice.
One more thing: Don't feel bad when you lose a team match. I'm guessing your friends aren't caring much. They want to play with you, so relax, man.