You are right in that the 3rd column isn't really necessary if the property was not disposed to spouse.
However, if you are preparing your return together with your spouse, your entries on the Principal Worksheet will only copy over if you indicate "Yes" to "Did your spouse/partner also dispose of this property?"
Just a final note, once you filled out the worksheet, you will need to go into the T2091 to complete that form. The Review will also prompt you to fill out any missing information.
