That's how it was that night. Genkei had to stay with them for now while they figured out what the hell was going on, but with the lack of space and futons, the three had to rearrange themselves to fit comfortably in the 'cozy' apartment. Saitama figured sleeping between the two Genos's would be the best option so that Genkei wouldn't freak out or Genos wouldn't accidentally hurt the teen in his sleep (because time travel was complicated and Saitama didn't want to risk anything). Plus he was already tired from all the drama that came with this kid, he just wanted the quickest solution that would end with him in dreamland.