I have 3 cells called "overtime" Traveltime"and "Servicetime"
Those cells are in a single row.
I am looking for a way so that only ONE of those cells are allowed to be
filled with the letter "J". So only one cell i allowed. When already a cell
is filled, the a message must be poped up like "already one value entered,
only one value is allowed".

I hope somebody can help me out here!!

thanks