I think the third person is Heer because lately we've been noticing a change in her behavior, all of sudden she's all chirpy and talkative which is really unlike her. The way she jumped onto the plane today and began questioning Prem, it seemed like she knew that she was foiling Gaurav's attempt to leave Ludhiana.
Another big thing, Prem and Heer have a mutual bond where they know eachother and udnerstand one another very well. So by now Heer would have seen through Gaurav and realized that he isn't behaving like Prem. If Heer hasn't realized that Prem is not himself then that would go against everything we have learned about Prem and Heer until now, so by now Heer DEFINITLEY knows that Prem isn't Prem (at the moment). So that third person can't be anyone else but Heer.
I really hope that it is her and that the creatives arn't going to introduce some new character because that would be stupid.
And along with Heer I think Preet is in on the secret too because he was at the airport that night that Gaurav was supposed to leave for Mumbai, so he must have seen something. And in yesterday's episode I think it was probably Preet who told Gaytri mom that Heer and Prem arn't living as husband wife. By telling her this, they would be able to get Heer and Prem to live in one room and that way Heer would be able to keep a better eye on Gaurav. I don't know I just hope the creatives actually have a creative enough mind to produce a track where there isn't unrealistic things happening like a new third person being introduced into the show.
In conclusion if Heer is the third person it would explain her change in behavior (her chirpyness), it would also explain how she found Prem sitting in the plane and how she found the tickets. It would also explain how she has already realized that Gaurav is pretending to be Prem.
Let's just keep our fingers crossed that this track doesn't get anymore ridiculous as it already is
-Riya