The number can be read or written out as
... "two and three tenths",
or as
... "two and three-hundred thousandths".
The latter form is tricky, because it needs to be distinguished from
... "two and three hundred-thousandths," which is 2.00003.
You would only use the form involving thousandths if you needed to ensure that the precision of the number were communicated properly.
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The value of the digit 3 is 3/10, three tenths.
