Hi, yes, you can select 3 random challenges, you can found all challenges on:
To make your challenge unique you must make sure that the language that you chose for the challenge didn’t exist on the folder for that challenges and see in the OTHERS.lst file that the language didn’t have already any external solution.
On the other hand, we have a template for post related to immersion/challenges if you have any new questions please follow it or you may not receive any answer.