This is my theory of 3 girls being buried "that night."
NOTE: Again, this is my own personal theory that doesn't make this true 의해 any means but I think this is my only theory that I have so far, that could end up being true..

1. Sara Harvey went missing around the same time Alison did.
(We find this out when Emily and Hanna go to visit Sara's 프렌즈 Claire and Tina)

2. Remember when Sara was found in the dollhouse? (Yes the same one where the liars were) The cop asks her name she said "I'm Sara, Sara Harvey" This mirrors when Mona thought she was Alison she said "I'm Alison, Alison DiLaurentis"...