How to instantiate a platform each time another platform is 3 units away from the gamecontroller?

I will explain my situation better

I want the top bar to set up gameobjects (platforms). Each time a gameobject that has instantiated this at a distance of 3 units (for example), the bar instants another gameobject, selected at random from a group of gameobjects (GameObject [] platform;), and so on. I know you can get a similar result by saying that a platform appears every two seconds for example, but the game speed will increase, so this script would not work as the platforms will be more and more separated.

I must also clarify that the platforms do not move, at least they do not change their position on the y-axis, but instead move the bar and the camera together, upwards, at a predetermined speed (the speed of play), this I do so because the score is based on the distance traveled by the player.

Also, that bar is something that is not seen in the game, it will not be rendered, it will only be a location on the y axis.

The gameobject will appear in the 0 position of the x axis ... contrary to the random position that appears in the image (I'll take care of that later).

All help will be well received, Thank you for your attention. :)

(translated with Google Translator, so that the question reaches more people)