In episode 20 of Attack on Titan, how does the Female Titan have 3D Maneuver Gear after regaining human form?
This answer pretty much explains everything.

Whilst riding on horseback


  Zoe- Erwin, why did you order Levi to resupply? There's no time to
  waste.
  
  Erwin- The female-form Titan was eaten. But did you see the person
  inside get eaten? I didn't.
  
  Zoe- You can't mean-
  
  Erwin- Ah, yes. If your original hypothesis was correct, they can
  still move to a degree after regaining human form. If they'd prepared
  3D Maneuver Gear in advance...





   Erwin- The person inside the female-form titan is wearing our uniform right now... The enemy is now camouflaged as one of the troops.