The instructions are here: https://www.strictlybedsandbunks.co.uk/wp-content/uploads/2023/02/18D-Duke-Triple-Sleeper.pdf
If you follow them, you will install 1 centre rail with the final support leg, halfway across the "bottom fascia."
If you follow these instructions exactly, you will not install the second support rail anywhere, nor will you install the slats in the top bunk.