Clinton included it in his 1994 State of the Union Address:
“Now those who commit crimes should be punished. And those who commit repeated, violent crimes should be told, ‘When you commit a third violent crime, you will be put away, and put away for good. Three strikes, and you are out.'”
He chose the slogan knowing its resonance with the US public as it had long been a baseball term and so familiar to voters there. The earliest citation I can find of the phrase in print is in the Ohio newspaper The Zanesville Signal, November 1940, in an advert for Dutro’s Motors:
“Three
Strikes
And you are out in
a ball game but
one strike
in a shimmying car and
you are out to stay.
Let us stop that
shimmy before it
strikes you out.”
[Note: shimmying is shaking or quivering caused by faulty wheel alignment.].
The nature of the advert makes it clear that the phrase would have been well-known to its readers in 1940.