Is there a better way of adding the latest 3 values in a row where the value is not 0 (i.e, skip column when value = 0)?   

Example :

```
       [Jan]      [Feb]     [Mar]       [Apr]      [May] 
       [2]        [3]       [10]        [0]        [7]
       [2]        [3]       [10]        [0]        [0]
       [2]        [3]        [0]        [0]        [7]
       [2]        [3]       [10]        [0]        [7]

```

Looking for the row values; 20, 15, 12, 20.

At the moment I'm only thinking of a massive case statement...