DevOps, CI/CD and Automation

Hi,

I've three placeholder columns like A,B,C which are populated by the procedure. Now i wanted to add another column D and this column needs to be populated by adding column A,B,C. I tried using D as a placeholder and try to assign the :A + :B +:C but now luck. I also tried D as a formula column as well but it didn't work. I don't want to use procedure to populate this because A,B,C alredy got the values. Please advise

thanks