If I understand you correctly, you want to sum all the values in a third column, when the values in columns 1 and 2 on that row are the same.
There are several ways to do this and here are two of them:
1. Use a fourth column to compare columns 1 and 2 and then use the SUMIF() function to sum the values on the rows where there is a match
B C D E
2 Data 1 Data 2 Values Match
3 A R 75
4 C T 45
5 G H 29
6 Y D 59
7 R R 95 X
8 W B 33
9 F F 18 X
10 G H 93
11 Y E 33
12 C W 10
14 Total 113
The formula in cell E3 is
then drag the formula down to the last row of data, row 12 in this example.
The formula in cell D14 is
2. Use an array formula. For this you don't need the fourth ('Match') column
The formula in Cell D14 is
To make this work as an array formula, after entering the formula, put your cursor in the formula bar somewhere inside the formula itself and then hit Control + Shift + Enter (all three keys together).
If you get this correct your formula will be surrounded by curly brackets
Entering curly brackets from the keyboard does not work and hitting the Ctrl+Shift+Enter combination when the cursor is not in the formula does not work.