What do you mean? If you want to create an object at the position y: 100 and random x: 100 or 200 or 300 you can do it like this:
every 1 seconds - system - create object at position y: 100 x: choose(100, 200, 300)
From the manual:
choose(a, b [, c...])
Choose one of the given parameters at random. E.g. choose(1, 3, 9, 20) randomly picks one of the four numbers and returns that. This also works with strings, e.g. choose("Hello", "Hi") returns either Hello or Hi. Any number of parameters can be used as long as there are at least two.