Why is mistress called " small/ little three"?

Hi everyone!!
Merry Christmas to all who celebrates this great holiday!!!
Today I am making a short post to teach you a fun word that literally can be translated as small or little three and it means a mistress. As you may see that means there is another girl or a third party in your marriage. If someone called you 小三，it always have a negative meaning and it is a derogatory term.

How does 小三 look like

• young
• pretty
• unmarried
• usually searches an easy life with man who can provide her with comfortable life.

Why is she called 小三?

It is simply because the wife is called 老大( old, big),
二奶 ( second wife/ mistress)， so what is left is 小三。