How can I get easy Plane kills in Battlefield 3?
Unfortunately there really is no guaranteed way to do this. BF3 has been out for some time so there are a lot of players who are ridiculous in those jets. There are a few things I could suggest though:
Play a map with multiple aircrafts: Any map that has more than one plane, and more preferably multiple helicopters will be a good place to start. Take down a helicopter and you can score some nice points for a double kill (maybe more) and vehicle destruction. Having multiple aircraft in the sky also creates more distraction for everyone involved giving you a decent chance to fly under the radar (literaly).
Play with some friends: Having a good squadmate or two can really turn the airfield in your favor. Spotting jets and helicopters makes a massive difference early in the game for jet pilots. Until you have the radar on your jet you are basically flying blind, and it is VERY tough to spot those choppers until it is too late. Also teammates can assist getting jets OFF of your tail with a little SOFLAM and Javeline action.
Play conservativly: Once you get that jet, don't go losing it. Once you die in the jet the chances of you getting it back in that same game are slim to none (depending on the fullness of the game and the number of jet hogs there are) Don't fly near the enemy AA weaponry and try to stay low to avoid locks (since you can't do anything once you're locked)
Don't rule out ground units: If your team is spotting, look for the humvees and DPV's. In a jet these are cake to take out and can sometimes provide 2+ kills and vehicle destroy.
All in all getting past those flares is the hardest part, it only gets easier from there. Score yourself a few good helicopter kills and you could be looking at the unlock in one or two sweeps.
There are a lot of 'flight school' or 'flying practice' servers out there, usually with player size around 4 people. You can join one of these, fly a plane around for practice, and dogfight with 1 or 2 other people, without worrying about ground or anti-air. You'll get the 3 kills you need to unlock flares in no time, and it's also just nice in general to be able to get the hang of flying without all the attention you attract from anti-air in a normal game.
To find these servers, just search by keyword 'flying' or 'flight' in your server filter.
Personally, I fly with a mouse - no joystick. The first unlock seemed to be the hardest, and obviously because you're only ability for evading a missile is by leading it into buildings, hills, etc or by turning left or right and continuing into a corkscrew downward at max speed. This WILL break the missile lock.
Also, this may sound cheap, but attack the enemy's uncappable base if the server rules allow it. When the enemy is taking off ( jet or helicopter ), they are most vulnerable to attack as they cannot evade your main guns. [ They can bail out, but you will still get points for destroying a vehicle ] KEEP IN MIND, you can get kicked and/or banned for 'baserape'.
Also, Check out this great video on a tutorial for jet flying.
This may sound silly and it will be slow but while in the air spot like crazy. Even if you can't hit the broad side of a barn, if you keep spotting you'll eventually get spotting points. If I recall it doesn't take a whole lot to get heat seekers so this is a viable way for people who have difficulty aiming.
Also, you can try ramming people on the ground. As long as you get them dead before you die, you should get roadkill jet points. Additionally, if you ram air vehicles, occasionally you'll get lucky and be credited for a kill (especially on vehicles that aren't 100%).
Again, these are unconventional and slow but if you find the learning curve very sharp, these could get you there so you have a few more tools to work with.