Pixar built Hollywood’s most consistent animation shop by balancing interest from families and adults. But that brand strength doesn’t always translate to 3D success: Last weekend, “Brave” yielded a 3D share of only 34% for its domestic B.O. tally — a new low for stereo toons.

That result can only stoke Hollywood’s concerns about the format’s place in animation, once seen as the vanguard of the 3D revival. While the gap between 3D toons and their live-action counterparts widens and execs seek to understand why foreign auds spark to the format more than Americans, they’re faced with a more ominous question: Are animated 3D films becoming an exception, or signaling a deepening domestic erosion?

It’s no secret that 3D shares have slowly declined for both family and fanboy fare. Recent toons by DreamWorks Animation and Sony Pictures Animation have outpaced Pixar in domestic opening 3D shares. DWA’s most recent offering, “Madagascar 3,” saw 45% of grosses from the premium format, but by comparison, the studio’s “How to Train Your Dragon” in 2010 commanded 67% of its B.O. from 3D — the highest for any toon since 3D began making its comeback.

Pixar has trailed its animated competition going back to its first 3D release, “Up,” which grossed 53% of its opening from 3D in 2009 (that same year, “Monsters vs. Aliens,” from Paramount-DWA, debuted domestically with a 58% 3D share; Sony’s “Cloudy with a Chance of Meatballs” rang up 60%).

While DWA’s Jeffrey Katzenberg has been at the forefront of the modern 3D resurgence — having vowed since “Monsters” to make every pic in stereo — Pixar chiefs have downplayed the format’s significance from creative to marketing. Around the time “Up” was released, Pixar stereoscopic supervisor Bob Whitehill said the studio takes a conservative approach with 3D, emphasizing story and emotion over depth and gimmickry.

Still, the company has released all four of its last films — “Up,” “Toy Story 3,” “Cars 2” and “Brave” — in 3D. Studio has even committed to re-releasing 3D versions of “Finding Nemo” and “Monsters, Inc.,” which weren’t originally envisioned for stereo, according to Disney.

The fact that Pixar falls behind in 3D shares — at the domestic B.O. at least — has some bizzers scratching their heads, since the studio’s films play extraordinarily well with teens and adults, even more so than the pics from other animation shops. Still, when it comes to families, parents are increasingly wary of paying 3D premiums unless word of mouth suggests that the format is highly additive.

Domestic 3D shares don’t tell the whole story, however: The majority of global 3D box office comes from international markets still hungry for it, particularly China and Russia. “Brave,” for instance, was released only in 3D in China, and it scored a 79% share from the format in Russia. Overall, 3D contributed 73% of “Brave’s” international B.O. haul to date, from territories that also included Australia.

It’s those overseas perfs that make the costs of 3D production or conversion worthwhile. Domestically, audience taste for 3D has fluctuated considerably over the past few years — and not just with Pixar pics.

The format has always faced a challenge in drawing families — pint-size moviegoers tend to tire of the goggles pretty quickly. But in 2010, DWA scored with “Dragon” and “Shrek Forever After” (which had 62%). Last summer, however, DWA lost 3D traction with “Kung Fu Panda 2,” at 45%. Disney-Pixar’s “Cars 2” earned 40% from 3D a few weeks later.

For Pixar, the “Brave” 3D performance is unlikely to cause any agita, as the Mouse House unit’s track record proves it’s not as reliant on the 3D bump as its competitors: “Toy Story 3” holds the worldwide record for highest-grossing animated film, with nearly $1.1 billion, while “Cars 2” rang up a considerable global B.O. tally of $560 million.