Data validation with a fixed amount of decimal places
 

I would like to force a user to enter a number that contains a decimal and
three spaces beyond (0.000). I think this can be done with the Data
Validation feature but I am having difficulty writing the formula. The
number of places to the left of the decimal can be limitless and the number
to the right could be less than 3, but I need to have the 'missing' places
populated with zeros.