Second Stage Theater is getting a third stage — which means another Gotham nonprofit is taking a Broadway venue out of play for commercial producers.

When Second Stage announced this summer it had acquired rights to purchase the Helen Hayes Theater in 2010, the move put the Off Broadway org on track to become the fourth not-for-profit to own or operate a Main Stem venue.

This added fuel to concerns harbored by some old-school commercial producers that nonprofits are encroaching on the Tony spotlight without producing under the same financial risk/reward model.

But the acquisition of the Hayes reps another example of the increasing interplay between commercial and nonprofit producers in recent years — and at the same time remains a large and potentially risky step for the 29-year-old Second Stage.

The org needs to raise $35 million to purchase and renovate the 597-seat Hayes, the Rialto’s smallest theater. And in case you hadn’t heard, this may not be an ideal time to launch a capital campaign.

“I don’t think it’s going to be easy,” says Second Stage a.d. Carole Rothman. “But I’m encouraged.”

If the sale goes through, Second Stage will become a Broadway compatriot of nonprofits Roundabout Theater, which owns Studio 54 and rents the American Airlines (and may acquire another space, Henry Miller’s Theater, in the future); Manhattan Theater Club, owner of the recently renamed Samuel J. Friedman (formerly the Biltmore); and Lincoln Center Theater, which owns the Vivian Beaumont.

The nonprofit profusion has brought its share of awards attention, including recent LCT hits “The Coast of Utopia” and “South Pacific,” which won seven Tonys each.

Such victories have, in some corners, prompted gripes that echo the Emmy grumblings from networks over trophies lavished on feevee fare: Because the nonprofits play by different economic rules, they can take more creative risks, which is perceived as giving them an unfair advantage when it comes to kudos.

Unlike commercial producers, who can take on millions of dollars of risk to put up a Broadway show — money that may or may not ever be made back — nonprofits raise cash each year (some of it coming from foundations, grants and the government) to fund an upcoming season and to sustain the org over time.

On the other hand, extra income from a hit also goes into the institutional pot.

“We don’t have quite the pressure Broadway producers do,” Rothman acknowledges. “But we don’t put the money in our pockets, either.”

The rising nonprofit presence on the Rialto reflects a larger trend that sees an increasing number of commercial and nonprofit collaborations, spurred in large part by the spiraling costs of legit development and production. It’s now commonplace, for instance, for Broadway producers to drum up enhancement money to supplement nonprofit productions, coin doled out in exchange for future commercial rights if the project proves a success.

Second Stage isn’t a total stranger to Broadway, where commercial incarnations of its productions of “The 25th Annual Putnam County Spelling Bee,” “Metamorphoses” and “The Little Dog Laughed” have played in recent years. If the company pulls off its fund-raising goal, the presence will expand into a dedicated Rialto space for its programming.

With the new capital campaign, Rothman and exec director Ellen Richard expect donors to be hesitant initially, only to return with more coin closer to the 2010 closing date as the economy drags itself out of its slump.

The theater also aims to up its marketing and aud outreach to augment the current subscriber base of 10,000, in order to help sustain an operating budget that will double to hit $14 million (up from $7 million) when Second Stage takes on the Hayes.

Adding a third stage wasn’t the initial intention when the theater first began discussions with the owners of the Hayes. The org’s lease on its 296-seat Midtown theater, classified as Off Broadway, was about to be up, and it seemed likely they would be evicted. (The lease has since been renegotiated, with another eight years to go.)

“Two years ago, we were looking at being homeless in 18 months,” says Richard. “Finding a permanent home was the goal.”

The intimate Hayes seemed like a sustainable size for the new American plays Second Stage aims to put in the theater.

Martin Markinson, one of the owners of the Hayes, also liked the idea, given the prevalence of musicals and Brit imports on the Main Stem.

“You need more plays and more American playwrights on Broadway to move the business forward,” he says. “What I like about the nonprofits is they can take risks, and they do take risks.”

Rothman characterizes the theater as the only nonprofit devoted to new work from U.S. scribes, which, in her view, does not put Second Stage in competition with the other Rialto nonprofits, whose programming includes a fair share of revivals and imports.

In addition to the Broadway and Off Broadway spaces, Second Stage also will continue its summer series of plays by emerging writers in the McGinn/Cazale, a 109-seat uptown venue.

Continuing to operate on a subscription model, the org plans to produce three shows at the Hayes alongside three at the Off Broadway space (down from four, allowing popular offerings there more scheduling room to extend). The uptown programming will continue with two shows every summer.

Rothman gives a thumbs-up to the increase in pay scale that comes with producing on Broadway, which will have Second Stage paying writers, actors and designers “a living wage,” as she calls it.

She also sees the Hayes as a logical next step in a potential pipeline for developing stage talent, with emerging artists bowing uptown, graduating to the Off Broadway venue and then perhaps moving on up to the Rialto.

In addition, Rothman and Richard see the upside to the leap in visibility that comes from a regular presence on Broadway — a profile boost both for the artists and for the theater itself.

“Broadway is like the Good Housekeeping seal of approval I’m happy to have,” Richard says.