That'll get him all decimals, not just the last three.

Assume we're dealing with 123.456789.

=A1-TRUNC(A1)
will result in 0.456789

=ROUND(A1-TRUNC(A1), 3)
will result in 0.457 (it rounds it off to three decimal places)

=(TRUNC((A1-TRUNC(A1))*1000))/1000
will result in 0.456 (this is a nasty formula, but it basically shifts the decimal three places and re-truncates)

=RIGHT(A1-TRUNC(A1), 3)
should result in 789 (though this one gave me a floating-point error when I tried it, which may mean it's bug-prone in practice and needs caution)

Which one you want depends on exactly what you're trying to do, but hopefully that should cover all the bases.