The three-star rating applies to a comparison with past series of this programme, not to television dramas in general.
This series takes different approach to those in the past - the story line is continuous, as opposed to self-contained episodes in the past. However the meaning of each episode is cumulative and so to understand what is happening in a current episode one has to have a good memory of what had happened in previous ones. This does create extra complexity, which for me is great, but could be irritating for others.
The major flaw in this series is that in an attempt to appeal to a wider audience, there is a greater emphasis on thrills and spills which sacrifices script quality and leads to very implausible situations and cliche endings. For example, in one episode Adam Carter is ordered to carry out "soft" surveillance on a subject going into a store. He does this by standing outside its front window, looking inside, with ear-piece in full-view. A 10 year-old could figure out this is not good spying! In terms of cliche, in one episode Harry does a lot of illegal things but is able to keep his job by turning up some dirty stuff on the Home Secretary - hardly original or exciting.
I loved previous series for intelligent script-writing, the raising of complicated problems with no easy answers, and presenting ethical dilemmas, again with no easy solutions. A lot of this has been lost in the search of thrills. The programme continues to be very enjoyable to watch, but I hope the producers reverse the latest trend in the upcoming series if they don't want the programme to become just like any other ho-hum thriller that does not require an IQ to watch.