there are 3 excel cols,

when user enters in col1, it should be allowed only if data is not
present in col2 and col3.

also when enters in col2, not be allowed to enter if data is there in
col1 and col3

also when enters in col3, not be allowed to enter is data is there in
col1 and col2

so which is the formula to use in the excel col so that the above
holds true.