Comes from the childrens nursery rhyme:

Number 1, tickle my bum
Number 2, do a big poo
Number 3, go for a wee
by Kellygirl February 11, 2012
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Rhyming slang for doing a pee or wee (urinate)

In the same way number 2 is rhyming slang for poo.
Had too much to drink, gotta go number 3.
by Jazpur September 04, 2011
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The act of performing an abortion on a toilet, flushing down the fetus.
Sally was in the washroom for a long time because she went number 3.
by Gerry McGee April 08, 2008
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Number 3 means full sex with a woman. Ask and agree = three - rhyming slang.
Men often visit prostitutes for Number 3
by Stias September 04, 2005
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To vomit into a toilet. Typically done after a long night of drinking.
"Gross...someone's number 2 is gonna make me go number 3!"
by MarshoT December 01, 2009
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The beer shits. Also known as pissing out your ass or shitting through a screen.
"Dude, I drank too much Mickey's last night and need to take a number 3."
by k-l-s August 13, 2008
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