So there are a number of ways to do this. You could have the server do simple RNG on the the 3 different values whenever the client sends it what choice(position) they chose. I will link the built-in RNG for the Godot engine below. This should be simple enough to figure out how to set it up if you want to use this route. If you have to create your own, because of how your server is setup, that is on you.
RandomNumberGenerator -- Godot Stable
You can then send back the data however you want, a simple way is to send a string of the path name of the image you have in project already. So you can load that into a Type:TextureRect that you set within the scene you want. If you are preloading textures(this is not a simple you would think) you can just pass the object in. If not you need to load the image then pass it in.
var currentimage:Resource = load(stringpathfromserver)
Hope this helps.