Here's why:
* There's no such thing as a three dollar bill. It's an imaginary currency, making it inherently odd.
* The phrase emphasizes the absurdity: It uses a ridiculous, non-existent thing to highlight the unusual nature of whatever is being described.
It's important to note: This phrase was often used in a derogatory way, particularly in the past, to refer to someone who is LGBTQ+. However, the meaning has shifted over time and many queer individuals now reclaim the word "queer" as a positive term of self-identification.
Therefore, it's best to avoid using this phrase. It carries historical baggage and could be considered offensive to some. There are many other ways to express "unusual" or "strange" that are less likely to cause offense.