SUm of table

You don't say how wide your different signals are...

Assuming z is 2-bits wide, y 3-bits and x 1-bit:

reg x;
always @ (*)
case (z):
  1 : x = y;
  2 : x = y + y;
  3 : x = y + y + y;
endcase

This would also work if x & y are wider - but the same width as each other - and y an array.

You'll need to specify what happens to x in the case of z == 0.

Regards,

Alex