Hi
you have two options -
1) format the cell to only show three decimal places (click on the cell,
choose format / cell / number - decimal places 3, OK). Using this method
the original value will still be used in calculations
or
2)round the number to three decimal places - in an adjacent cell type
=ROUND(A1,3)
where A1 is the cell reference of your number. You can then copy it, click
on the original number and choose edit / paste special - values to replace
the original number with the result (and then you can delete the other
cell). In this way, the "rounded" value will be used in future
calculations.
Cheers
JulieD