calculated column - days difference calculation

Hi,

Please i would appreciate help in calculate the total number of days in a sharepoint column

Once an item is created in sharepoint and before column 1 is populated, the the time to resolve says "Not yet calculated"

As soon as column 1 is populated, the time to resolve gets populated.

As soon as Column 2 is populated, the date difference between columns 1 and 2 is recorded in the Time_to_Resolve Column. 

As soon as column 3 is populated, the date difference between columns 1 and 3 is recorded in the Time_to_Resolve Column. 

Thank you in anticipation