It's on Netflix now, but DVD-only. There are only six episodes, and they're only a half-hour or so long, and I've watched the first four (i.e. the first DVD). I just did a search to make sure I was right about the number of episodes, and saw a sad headline: "Why Detectorists series 3 will be the last." So now I will never have more than two episodes yet to watch. But I will give writer-director (and lead actor) Mackenzie Crook the benefit of the doubt, and suppose that in his estimation he can't do more with the characters and still keep up the quality.
Anyway: if you've seen the first two series and liked them, I'm sure you will like this one. If you haven't seen them, you should. I might rate this one of the best things ever done for television. I just read over what I said about it in December of 2016, and it still holds, so I won't repeat myself.
I will however repeat that it's really sad that there will be no more.
Oh, and one other thing: I did notice one aspect that seemed a falling-off from the previous two series, and that was the quality of the cinematography. It isn't as crystal-sharp and vivid as I remembered. Then I realized that I had watched the other two in streaming HD, and basic DVD quality is lower. So I guess there's a reason to have Blu-ray.