Try to seperate the ufo's by letting them chase your aircrafts. If they're all chasing after one of your planes, then use your other flyers to flank them. If they go for a 1 vs 1 approach things get a little trickier, but try to seperate them in a way that one of your aircrafts can flank one of the UFO's chasing one of your other aircrafts (basically send your planes off in different directions until the enemy is seperated, then turn one of your planes around so that it can chase after another UFO), the second aircraft (now alleviated of it's persuer) can then return the favor by flanking the first crafts persuer. Both of them can then easily kill of the last UFO from the rear while it keeps chasing your third plane.
I don't know how understandable that explanation is, but that strategy has been working for me so far. Haven't gotten very far into the game yet though, so it might not be a viable strat later on.
I used to get my ass handed to me all the time aswell. Because the old system was so simple, I didn't think that you were meant to control the combat in such detail, so I just let them attack the UFO's head on and tried to survive by using the evasive manouver-thingy. Worked in the beginning, but I soon lost at least one plane in every fight, so my economy got broken and I'd start getting my ass handed to me on the ground aswell. Creates a really steep downward spiral when you start losing aircrafts.
