reveal final colum once data in previous colums are entered

[nlt]

Hi all,
I want to create a spreadsheet with 4 columns. The fourth colum will be complete with values in but the first three empty. I want the value in the fourth column to only show once data has been entered into the first three columns. Can anyone tell me how I can do this?
Thanks

[Special-K]

in D1 (fourth column)
=IF(LEN(A1&B1&C1) < > 0,value,"")

where value is your required result