In my Results, for each event, I can only get field A OR field B.
I created two aliases: A=C, B=C
So each event should have C with a value.
But in the result, I can see that sometimes C gets value A, sometimes value B, and sometimes Blank.
If you have only one of the three field with value or if you have an order of importance in your fields, you could use the coalesce function:
| eval C=coalesce(C,A,B)
in this way your C field take value, in order, from C, A, B.