2v1 and 3v1 games are rather rare to happen, but once in a while they can occur.
They are usually played in 3 players maps, and usually the player that is alone is higher skilled than the other two (the difference should be high enough, or it will be hard for him to win against 2 or 3 players). In some 2v1 or 3v1 games some rules can be set (like "1 shaman life", or "no rushing", or "if the alone player doesn't lose after x minutes, he wins the game".
In the past few years, 2v1 and 3v1 maps were created, so this rules wouldn't need to be set, and more 2v1 and 3v1 games would be played. While there are many 2v1 levels, the most known and 
played 3v1 level is "Team Nashua Worlds' Juggernaut". In that level, red fights the other three tribes by himself, but he has land, wildman and mana totem pole advantage. This makes the level 
rather fair. Still, it is believed that if all the 4 players have similar skills, the 3 allied have more chances of winning that level.
Also, 2v1 and 3v1 games can be good for getting skills, if you are the one fighting many players  alone (even if you lose, you will feel stronger when you play 1v1 or 2v2 games).