It sounds like you are deliberately putting yourself into 1v2 fights while you are not yet ready for them. In order to come out of a 1v2 fight successfully, your 1v1 fights need to be nearly flawless. A 1v2 is not merely twice as hard as a 1v1 - it is orders of magnitude harder. Whereas in a 1v1 you can trade hits or take the occasional hit and recover, with multiple opponents you don't have that luxury. You don't even have the luxury of using your high cooldown power finishers (Luigi's up B or Fox's up smash) because the other guy can punish you for killing his teammate. If either one of your opponents is evenly matched to you the fight will be nearly unwinnable.
Now assuming your 1v1 skills are good enough, you still need to fight strategically to win. Unless friendly fire is on, you don't want your opponents to be grouped together - their hitboxes will just stack and there will be very little you can do.
Your priority is to separate your opponents, and then get K.O.'s as efficiently as possible. Typically this means targeting the weakest of the opponents. Use the barest minimum amount of effort to keep the stronger opponents from getting too close (toss projectiles their way or a good spacing aerial attack - back aerials work fairly well for Luigi and Fox) while you try to go for low-percentage gimps on the weaker opponent.
Also, pick a large stage to give you room to escape when you need it. Hyrule Castle or Sector Z are good candidates. Maybe Zebes if it is allowed and just try to be king of the top platform and let the acid do the K.O.'s for you.
If it's a free for all stock match, just run away from everything - last survivor wins. You have no incentive to actually get k.o's in that mode. If it's a time match, just treat it as a team game with friendly fire on and target the weakest player as described above. Use projectiles to steal K.O.'s from people by hitting people already hopelessly off stage.
Other tips:
Focus on dodging, spacing, and positioning. You want to avoid shielding or rolling if at all possible - even in a 1v1 a single opponent can put quite a bit of pressure on a shield. With 2 opponents hammering on a shield at once, you are just going to get stun-locked until your shield breaks or someone grabs you and throws you.
Abuse throws as long as you are not Link, Samus, or Yoshi. Not only are throws one of the fastest attacks, they work great for positioning your opponents (e.g. to separate them), and most throws turn the thrown opponent into a projectile which adds additional protection.
No amount of button-mashing can save me in those situations
I hope button mashing isn't how you got yourself in this mess in the first place...