>>9288604The second is indeed a curved blade.
>>9288610Those are all just variations of the first and second (straight and curved). (Also, before anyone says it, deactivatable blades like lightsabers don’t sheath in the first place, so they don’t meet the riddle’s specifications.)
>>9288609Fuck off. I’m a smart-ass, not a hack.
The third blade shape is a real shape. Albeit not the most practical one to use.
So as not to spoil it for anyone who still wants to guess, the answer is depicted in this ironically technically inaccurately named Oglaf comic:
https://www.oglaf.com/cannotbesheathed/ 