A three dollar bill is a slang term for something that is not quite kosher or correct.
Dude 1: Hey, want to buy a Rolex? Really cheap!
Dapper Gent: It looks as real as a three dollar bill. You stuck a piece of tape to put "ROL" over "TIM". That's a Timex, not a Rolex.
During the fun-and-tumble days of the 1990's, a common joke asked whose picture appeared on a three dollar bill. The answer was everyone's favorite Presidential Punchline, Bill Clinton.
Eventually, the three dollar bill became part of pop culture, including being part of Limp Bizkit's first album, Three Dollar Bill, Y'all.