Magnum has aged much better than other Donald P. Bellisario shows and other action dramas of the 1980's (like A-Team, Simon and Simon, Battlestar Galactica, etc.). Perhaps it's the "exotic" locale (and the fact that Hawaii today looks very much like Hawaii of the '80's). Perhaps it's the performances. I think it's really because of the timelessness of the episodes - Magnum is so firmly grounded in its own history that it is a complete universe and each episode further strengthens the series as a whole. With this season, the full regular supporting cast is filled out with the first appearance of Carol, the District Attorney (although she is not played by the regular actress who, confusingly, does appear in another episode as a guest star).
It takes a while to build up a fictitious universe, which is why seasons 1 and 2 of the series are curiously normal - I don't think anyone watching the average episode from those seasons would understand the huge cult following the series has. But, little-by-little, each episode added to the mythos, and it all pays off starting in Season 3. There are many good episodes (and, to be sure, some dreadful stinkers - noteably "Two Birds of a Feather" and "Basket Case"), but all of them are rooted in the histories of the characters, the history of the Hawaiian Islands, or both. Certainly, a big hurdle was overcome by the producers of the show when they refused to succumb to the obvious gimmicks of the show - the Vietnam vet angle, the Ferrari, the locale - instead incorporating the "gimmicks" into the fabric each and every episode. All the good shows of the series (and of this season) could only be told by this show. They include:
Vietnam vet stories: "Heal Thyself" - a nurse Magnum knew in 'Nam is accused of murdering patients, and herself suffers from post-combat stress disorder; and "Did You See the Sun Rise?" - Magnum confronts a sadistic Soviet-Vietcong liaison officer he met while a PoW in North Vietnam.
Hawaiian Island stories: "Forty Years from Sand Island" - a murder at an internment camp for Japanese-Americans in 1942 has modern-day repercussions; and "Almost Home" - a grieving daughter tries to scatter her father's ashes at the Arizona Memorial, only to discover her father was courtmartialled for being AWOL on the day of Pearl Harbor.
Other history of the characters stories: "Black on White" - while Higgins is recovering from his wounds, his unit commits a massacre in a Nigerian uprising in the '50's, and a survivor is trying to exact revenge; and "Faith and Begorrah" featuring the first appearance of Higgins's illegitimate half-brother, Father Paddy, the drunken Irish priest.
The other hallmark of the Magnum series is the combination of heart and comedy - even the most dramatic episodes have moments of comedy, and there are a number of purely comedic episodes that contain a surprising amount of heart. Cases in point: "Flashback," where Magnum literally dreams up the solution to a case he's working on by transporting himself (in his dream) to 1936. Similarly, master ham-meisters Ernest Borgnine and Donnelly Rhodes (best known to Canadians from DaVinci's Inquest, Danger Bay, and Soap) guest star in "Mr. White Death" and "...of Sound Mind" respectively, two shows that can only be described a comedies.
Overall, most of the episodes are good, and those that aren't still have something about them that helps build the overall ambiance of the series. It's certainly the best season so far.