Twist ending: they all got their cutie marks at some point but hide them like this.

@MezzoDragon  
You know, that could actually be a good idea for an episode: one of the Crusaders gets her cutie mark on her own and ahead of the two others. At first she’s overjoyed, but then she comes to think about the potential consequences for their friendship and fears a breakup (I’m considering Apple Bloom for the role, where she’d have come to consider herself as a mediator between Sweetie Belle’s girliness and Scootaloo’s tomboyishness – possibly emphasized by the breaking of an argument in the episode’s cold open – and got in her head that the CMC would irretrievably split without her). So she hides her mark, like Scootaloo here, and pretends to keep crusading with them so they can still hang out together. However, many incidents descending into rows eventually make her realize that ignoring her talent has only made things worse for all three of them and she finally decides to come clean with her two friends.
 
Just an idea I had, doubtful that’ll happen like this. However I’d be ready to bet that they’ll get their cutie marks in three separate episodes and each of them will have a different approach about their own self-discovery.

or she couldn’t stand to have a non-chicken cutie mark…

@Background Pony  
She’s paining it over to keep her friends.

So is she painting a cutie mark, or painting over it?

Wow… thats sad.