Sum values of columns into adjacent column

In the top column S cell where you want the result, enter the formula =SUM(T1,U1,V1) replacing row number "1" with the row number on which you are entering the formula.

Then, copy (ctrl-c) the cell containing the formula, and paste it into the cells in column S below.

When you copy & paste a formula, the spreadsheet will automatically adjust the row numbers according to where the formula is pasted.

Note, it will also adjust the Column letters, but in your case this is not relevant, since your Column positions are constant.

See the SUM formula documentation here: https://support.google.com/docs/answer/3093669