You won't find 1/3 games online but they are quite common in live casinos, usually in place of 1/2 games. The first number represents the amount of the small blind and the second number is the amount of the big blind. As you can see in the comments below - the skill level will vary depending on where you play.

The $1/$2 notation means the small blind must post a forced bet of $1 before each hand, and the big blind must put in $2.