From The Fall's Wiki page -

 

In September 2016, Cubitt confirmed that the third series is Dornan's last, but expressed intention for future series. Cubitt added that the fourth series "isn't going to be straight away."Anderson stated: "I'm excited by the idea of potentially revisiting it in a few years, to see what transpires in Stella's life afterwards."

That finale was certainly a brilliant episode but I feel like this series as a whole has been a huge disappointment and an example of where a series has gone on longer than it should have. Aside from that final episode, nothing really of note developed, only that it continually reinforced the message that Paul Spector is an incredibly dangerous yet intelligent man. I think the full story could have been done in two series. There's a slow build and then there's five episodes of nothing happening apart from Paul faking amnesia.

I am inclined to agree with you. I power watched the first two seasons after being recommended the show by a friend, I thought it was brilliant - but like everyone else, had to wait quite a while for the 3rd Season.

 

This season was just... so... boring.

The actors were brilliant, but, the story was just so dull. There was so much they could have done and so many directions they could have went. It just seems they took the most boring route possible.