Don’t know why, but third-row seating in sport-utility vehicles has become de rigueur.

Take the Chevrolet TrailBlazer, for example. The new midsize companion to the compact Blazer began life with two rows of seats to hold five people, but now a third row has been added to transport up to seven folks. The minibus version was given a new designation, TrailBlazer EXT.

You have to wonder what’s the attraction of three rows of seats and seven passengers. Perhaps automakers have added so many cupholders that they need more seats to put the beverage holders to use?

Russ Clark, TrailBlazer marketing director, said the EXT strategy is a “commitment to families that need more room” and that “extra space means extra friends–and they can be larger than kid-size friends.”

If you’ve traveled more than 50 miles in an SUV with kids, adult companions would be a welcome relief.

But offering a third seat in an SUV is one thing, offering a third seat without asking passengers to hurdle the second row to get to it, is another. The third row in the EXT we tested is well-executed.

To get in back, you yank the handle and the second row seat back folds flat against the seat bottom cushion. Yank again and the package flips over onto the back of the front seat to expose a wide aisle for entry. You still must be athletic enough to manage the high step-in height, but once foot has reached floor, the task becomes simple. Opting for running boards at $375 helps.

Clark’s boast about adults in the third row proved true. Too often the third seat is big enough for only kids or short adults who sit cross-legged. The EXT offers a deep footwell to plant your sneakers and a huge bulge in the roof to provide ample room for your melon. You won’t see the bulge from the outside of the vehicle because it is camouflaged by a luggage-rack extension. Clever.

Second- and third-row seats can be folded flat for more cargo capacity, though it would be nice if the third row could be folded from the cargo hold and not require you slip in the cabin to do it.

The extra hauling capacity comes in handy, considering the cargo space behind the third seat is snug. Golf clubs will lie flat but don’t expect to fit luggage for seven.

To make the little space seem like a lot more, EXT offers a movable hard shelf. Toss the clubs in, snap the shelf into the walls directly above the clubs, and you can hold a few more items. Hooks in the shelf allow you to hang plastic grocery bags from it.

Other than the new third row, what sets TrailBlazer EXT apart from the TrailBlazer minus the initials are the dimensions. The EXT is built on a 129-inch wheelbase and is 207.8 inches long overall. Its smaller sibling has a 113-inch wheelbase and 191.8-inch length.

You may ask, why an EXT when you have the full-size Tahoe with its 116-inch wheelbase and 198.8-inch overall length? Or, why Tahoe?

“We did a lot of research and found there are those who prefer Tahoe because it is 4 inches wider and provides more of a feel of spaciousness, while there are those who feel with those 4 inches the vehicle is too big. So we came up with vehicles for everyone,” said Cheryl Pilcher, marketing manager.

Because some want small SUVs, Chevy still offers the subcompact Tracker and compact Blazer. But it drops the two-door Blazer for ’03 and the four-door for ’04 and adds a Blazer-size replacement for Tracker in ’04 called Equinox that’s built off the Saturn VUE platform.

But we digress.

Pilcher said Chevy expects to sell more than 200,000 TrailBlazers for ’02, about the same as Tahoe sales of 202,319 last year.

Since EXT is only now arriving at dealerships in numbers, Chevy will split production evenly between the five-seat and seven-seat versions and then adjust output based on which is in more demand, she said.

Like the TrailBlazer, the EXT is powered by GM’s new 4.2-liter, inlin -cylinder engine that develops 270 horsepower, produces 275 foot-pounds of torque and allows it to tow up to 6,400 pounds–though to get that muscle you have to accept a 15 m.p.g. city/20 m.p.g. highway rating, which explains the 25-gallon fuel tank, seven gallons more than in the regular TrailBlazer.

To ensure EXT can tow even heavier loads, this fall it will add an optional 5.3-liter V-8 that develops 280 h.p. and produces 320 foot-pounds of torque. No mileage rating as yet, but have to suspect the V-8 will open Chevy to charges that yet another gas-guzzling SUV is draining oil reserves.

“The V-8 fuel economy rating won’t be dramatically different than that with the 6-cylinder. But some people feel they need a V-8 for the added torque for towing,” Pilcher said.

While the EXT with a 6 will tow up to 6,400 pounds, the EXT with a V-8 will tow up to 7,200 pounds, she said.

The 4.2 delivers ample power off the line and when pulling out to pass.

And the longer wheelbase provides a rather pleasant ride because it keeps occupants farther from the point the wheels meet the pavement–except in the third seat directly over the rear axle. So you experience few if any annoyances over rough spots in the road–except in that third seat.

When it comes to handling, the added 4 inches in width and wider tracking make Tahoe more limber and sure-footed than EXT, which starts to feel heavy in the wheel midway into corners and turns. The EXT feels even heavier if you approach the corner or turn in earnest. The body is prone to lean a lot until you ease off the accelerator–or tell your six passengers to lean the other way for balance.

Perhaps the addition of on/off-road 17-inch all-season radials ($140) on the test vehicle rather than the standard on-road 17-inchers led to some of the anxiety in corners at speed.

TrailBlazer is offered in LS, LT or LTZ trim, but EXT only as an LT with a choice of two-wheel- or four-wheel-drive.

We tested the 4WD version, which starts at $33,010. Standard equipment includes automatic dial-up transfer case, four-wheel anti-lock brakes, dual front/side-impact air bags, air conditioning, AM/FM stereo with CD player, eight-way power driver’s seat, tilt steering, fog lamps, power locks and mirrors, power side/rear quarter windows, automatic headlamps, OnStar emergency communications system with next-generation hands-free calling and 17-inch all-season radials.

The test vehicle also came with the optional $1,860 leather/convenience packages. The leather package includes leather interior, power driver/passenger seats, driver info center, automatic climate control, rear-seat radio controls and steering wheel controls for ventilation and audio. The comfort package includes those items and adds memory driver’s seat and outside mirror, turn signals on the outside mirrors, rain-sensing wipers and body-color side moldings and door handles.

Options added includ e a DVD player to keep the kids content on long drives at $995; and the leather/comfort packages. You can add a sunroof for $800.