Mike,
Look at Data Validation==Number and limit values to 100 to
999. You can your own "Error Alert" message.

Would this be OK?

"Mike Smith NC" wrote:

I have a cell that users enter a 3 digit number and another a date, both are
used to generate a serial number. Is there a way so that if a user enters
more than 3 digits an error message comes up and it doesn't update to the
serial number cell? Thanks for any help