Fighting multiple cons takes a mix of skills, knowledge and luck. There are very few who can walk into a 3v1 or higher with a decent chance of winning every time, much less surviving. The times I've won under those conditions were predicated on the following factors.
1. I flew a plane I had absolute confidence in. (The FM2).
Knowing your bird means more than flying competently. It means you know exactly when she'll stall, what she will do at any given alt, e state, or configuration including flap settings and when missing pieces. It also means having a plan for defeating every other bird in the game and being able to fly and maneuver while looking out each window view.
2. Prioritize targets in order of threat levels. In an FM2 that means dodging the bnz guys and killing the turn birds first. In a mustang it means killing anything that might catch you and then knocking off the pesky Brewster last. Remember threat levels are always subject to change.
3. Fly to your strengths and make them fly against theirs. Force the jugs to turn, force the brew to chase, force the yak to shoot and miss.
4. Steal every advantage from sun, and terrain. Who cares what they say about you on 200? You're outnumbered. Scrape them off trees, blind them by flying into the sun, hide in hills or limit their options by fighting in a steep valley.
5. Survive first, shoot second. Don't push for a shot if it's going to expose you to another con. The longer you live the better the chances that they'll make a mistake.
With regards to SA, there are exercises you can do to improve.
1. Up your favorite ride and practice doing rolls, barrel rolls, immelmans, etc. all while staring out your rear view. Do the same looking up, rear up, and every other plane view.
2. Every time you arrive at a furball, take whatever time you have to assess the e state of every plane, friend or enemy. Make note of the ones you can't easily identify, and be especially wary of them. Some birds like corsairs can be tricky. If you can, fly around the edge of a fight for a few minutes and observe how the relative e states change. I will usually stay out of low fights if my side has a distinct advantage, and that can afford the opportunity to learn e states without trying to fight at the same time.
3. Film it. Study it. Watch it from external and internal views and also from the point of view of your adversaries.
Hope that helps.