Personally, I've always interpreted this as a complete Cosette fabrication. She was busy snogging Marius but she needs to cover for the fact that Valjean heard Éponine scream, so that's what she comes up with. "Oh, I saw three burglar-type guys!"
She'd have been better off to have said she'd seen a rat, given that this is what instigates Valjean to pack up and go to England.
"Quand vous aurez besoin de Bahorel, capitaine, Bahorel est là! Je sais faire trébucher tous les chevaux du garde-corps avec une ficelle... Rien qu'une petite ficelle. Enfin, pensez à Bahorel du Café Musain!"