For all those confusion regarding the identity of Lian Cheng and Tan'er, they are all the same person for all the 3 seasons. The drama is very straightforward so if you understand the concept of parallel worlds, everything is very clear and easy to understand.
Mo Lian Cheng is the same from S1 & S2. Except this time, he is with amnesia. But he is still the same person. Young Master Lian Cheng in this new Dong Yue is the one who drawn. I guess we will never got to know him since now Lian Cheng from S1 & S2 has occupied his body in this season. While Sir Lian Cheng the doctor is Mo Lian Cheng's past life. Mo Lian Cheng has his memories as well when he went to Xianlung Continent at the end of S1. So, he is like 3 in 1 = Original Lian Cheng + Sir Lian Cheng when he got all Sir Lian Cheng's memories + 8th Prince when he combined with his body in S2. They are all the same person.
Qu Tan Er is the lady from Qu Family who is in love with Mo Yi Huai. Qu Xiao Tan went inside her body and she died at the end of S1 so this Lady Qu is no longer in the equation. S2 Xiao Tan is without memories but she got her memories back when she was kidnapped by her father at the alter and the orbs made her remember all back. That is why in the ending of S2, she mentioned Xianlung Continent to Lian Cheng to show to us that she has gotten all of her memories back. So at the end, S1 & S2 & S3 Xiao Tan is the same person. Tan'er is Qu Xiang Tan reincarnation and in this Dong Yue's timeline , Qu Xiang Tan did not die and establish Dong Yue instead of Mo Yi Feng, That is why only Lian Cheng is reborn this this timeline (Young Master Lian Cheng from Mo Family) and not Tan'er -- the body that Tan'er occupy this season just happen to have the same face as the Empress since they are Aunt & Niece but she is NOT her reincarnation as Qu Xiang Tan did not die in this timeline.
