The original “Terminator” was eerily effective for its chilling Harlan Ellison-inspired story and its nonstop tension. “Terminator 2: Judgment Day,” seven years later, offered fans a spectacular quantum leap in special-effects technology, introducing the awesome “liquid-metal” scenes in which shiny, molten liquid “morphs” into a walking robot. “T2” also provided another landmark in sequelesque history: Schwarzenegger doing his granite-jawed best to play lovable, seeing as now his terminating-cyborg has been reprogrammed as the good guy.

Now here’s “Terminator 3,” and though deftly directed by Jonathan Mostow, it does little more than keep our interest. The “liquid metal” is back and still looks amazing, but the filmmakers are far too stingy with the effect, only showing us quick bits of it. Not once do we see an entire body morphing, just various parts of one. Not that there aren’t a few delicious, effects eyepoppers — at one point a robot arm reaches through a car seat, through the car’s driver and then drives the car— but mostly, “T3” is overstuffed with car chases, bullets and unlucky folks being tossed onto concrete.

Instead, the film’s biggest innovation — and THE reason to catch this third installment — is its newest cyborg, the “Terminatrix.” T-X, whose human form is that of a beautiful blond woman (newcomer Kristanna Loken, cast in the plum role no doubt for her intense stare). T-X is the most sophisticated killing machine yet — she shoots, stabs, spins her head around like Linda Blair in “The Exorcist,” and goshdarnit — looks terrifyingly terrific in red leather.

T-X has been sent from the future to find and eliminate John Connor (the appealingly soulful Nick Stahl, from “In the Bedroom”) who helped prevent Judgment Day in “T2.” Enter the original Terminator, whose obsolescence has been reprogrammed and upgraded to help John and Kate escape and save the world from the meglomaniacal machines. (Yes, one could pose the question of just why the fate of the world would hinge on an old-model cyborg? But trust me, don’t worry about it — the movie certainly doesn’t.)

Loken and Schwarzenegger make interesting adversaries; incredibly, she’s even more impassive than he is. (At one point, Schwarzenegger’s Terminator shuts down — and sadly, we in the audience must be told this because his expression doesn’t change one iota.) As required by “Terminator” tradition, the two “machines” engage in a climactic smack-down set in a futuristic men’s room, slamming the soft- and hardware out of each other against ubiquitous urinals (which is a rather odd setting for a “gender-free” final battle for supremacy.)

Despite the darkness of its subject matter — which is always a definite plus in my book — “T3” has more than a bit of humor, most playing off Schwarzenegger’s 2-ton way with a line. His final vocal foreshadowing, as usual, leaves the door wide open for a sequel.

So, will this once-a-decade “Terminator” pattern continue, with the hero collecting cyborg Social Security next time around? Or will it all hinge on the Gov. Gray Davis recall movement afoot in California? “We’ll meet again,” the Terminator tells John, but you know, the way he delivers the line sounds almost more like a gubernatorial threat than a promise. *** S