Create a random number generator. After that before the add_child statement wirte var x = random number generator.randomise(range here probably or whatever the syntax is, check out the docs)
After that in nxt line type instance.translation.x = x
In next line type var y and reapeat it for instance.translation.y and translation.z if you have it or if you need it.
If you didn't understand properly, check out godot docs for RandomNumberGenerator and you will get the idea of what to do