Well the solution is in that thread but if you got minimal linux knowledge its not so easy to provide efficient support but if you settle on OSMC i suggest that you do a clean install without noobs using our installer.
if you still insist on trying right now, i can redirect you to our wiki Wiki - OSMC then its just a matter of trying see if you can grasp the information from that thread if you still want 3 different OS on one SD
also for basic linux commands check out this link
best of luck 