Creating an if-statement in one cell that works on an entire column

I'm trying to create an if-statement with multiple variables. I want the if-statement to be typed only once, and to contain multiple parts. For example, say I have an if-statement similar to this:

=if(a1="x1","1",if(a1="x2","2",if(a1="x3","3", "")))

and I want to type it only once in cell a0, and it applies on the entire column with only typing "x1", "x2", or "x..", without returning the value of the respective variable in the respective cell. I was wondering if there's a way I can do such thing in Excel.

cell     a             b             c
0     columnx       columny       columnz
1     x1,x2,x3      x1,x2,x3        x*y
2     x1,x2,x3      x1,x2,x3        x*y
3     x1,x2,x3      x1,x2,x3        x*y
4     x1,x2,x3      x1,x2,x3        x*y