Who is the third A in Pretty Little Liars?

Answer 1

It is probably Alison (the real one, not Courtney, who died instead of Alison)
We already found out in S2E20 unmAsked that A was Mona. Then we find out that Toby is A. Then, Emily remembers what happened That Night and the highest ranking A is the blonde girl in the red coat. Now, there is only 2 A's we know of because Toby is now dead
Allison (the real Allison is the new a) If you are confused i will explain spoiler alert See Allison has a twin sister Courtney so one day Courtney goes down stairs to talk to ally's friends but they all think she is alli. Courtney lives in a mental hospital so when Courtney was visiting her mom hauls away alli, which who she thinks is Courtney. She tries to explain she is alli but they think it is Courtney tricking them again since she is crazy. So alli lives in the mental hospital and is mad that Courtney befriended all the pll girls, alli thinks they are all losers. So she is mad at her so she kills her sister so they all think alli is dead but, it is really Courtney. Sorry it is supper confusing

Answer 2

the third a is Spencers older sister, Melissa

Answer 3

There are only two A's: First it was Mona, and then it is Alison. actually there is mona,toby and alison.They are all the A's