It does not matter which you pick.
However since there is one times x in each it is easy to eliminate x.
x + y = 1300
x + 2z = 1400
-------------- subtract
y - 2 z = -100
then do the first and the third for example
x + y = 1300
x + y + z = 1600
---------------------
subtract
-z = -300
or z = 300
That was just lucky that there is no y term
If there were we would use the two equations left with y and z to get rid of one or the other.
now with z = 300 go back and use the one in x and z to get x
x + 2(300) = 1400
I think you can take it from there.