For al those retar- I mean, people - who mistake number 3 for number 1, it's a rhyming slang. Number 3 means to pee. Number 1 means none.
Little boy Johnny, went to the loo,
He ate so much, he went for a number 2.
Little boy Johnny, got out of the loo,
He drank so much, he suddenly had to pee.
So Little Johnny went there and gone for a number 3!