The main issue of S03 is that they practically solved the case by the end of Ep7. Going into the final Ep8, we do not know what keeps Wayne Hays going after this investigation in 2015.
That was not the case in S01, we knew that the case was not solved and murders kept occurring along the Louisiana coast, and that is what drove Rust and Marty to keep digging.
Here, in S03, we have no idea what keeps Hays after this case in 2015. The child is alive, there is no need to bring her home, there are no additional murders, Hoyt kinda took responsibility, child was in his home. Hoyt is dead, parents are dead. What is the purpose of investigation in 2015? We do not know. This is the problem with the script! This is why S1 is better than S3.
So the best we can say is that Hays knows something that the audience does not, or is simply going on sixth sense, that keeps him digging for solution, to something. We do not even know what he is after. Whether it is a grown Julie Purcells, or Lucy's and Dan's murderer, or whether he is simply after information to write the second book in honor of his wife. We do not know, and that is the problem with the script. Why are we in Ep8?
Now, just like in Ep8 of S01, there will be a breakthrough in the case, a green-house clue, that solves it all, even though we do not know what we are solving other than filling extraneous pieces of info, and in spite of the previous three episodes telling us that the case is practically solved it will be solved again, but differently. Almost as if the past three episodes were a waste of time. It's weird. And we do not know what kept them going to discover that last breakthrough clue unlike in S01.