The 3 is in the one-thousandths place.
You can call this number "zero point zero zero three."
Since the 3 is in the thousandths place, this number can also be called "three thousandths."
Use the name of the number to find the ratio.
"three thousandths" means 3 out of 1000.
The ratio is 3:1000 or 3/1000.