Conditional Formatting a Button
 

I have a spread sheet for reservations. Each reservation has 3 columns for entries, and a 4th hidden helper column.. For the most part 3 columns is enough,, however sometimes there may need to be more. I would like to create a macro that when all 3 columns have been used a button will appear above the 3 columns to add an additional column be it 4th, 5th or whatever. Is this possible? any suggestions would be appreciate
Thanks in advance!!!