Imagine a metal version of the Rat Pack - that's Trinity, a new vocal trio featuring some of metal's most notable singers.

"It's kind of like, in a way, what Sammy Davis Jr., Frank Sinatra, and Dean Martin did back in their Rat Pack days, except we're wearing leather and screaming a lot," Geoff Tate, the former leader of Queensrÿche who now fronts Operation: Mindcrime, said in a telephone interview recently.

Trinity, which also features former Judas Priest lead singer Tim "Ripper" Owens and former Iron Maiden front man Blaze Bayley, was formed after the three singers got together for a song on Operation: Mindcrime's September release Resurrection, the second record of a planned trilogy.

"We had such a great time hanging out, we thought, 'Why don't we try to put together some dates?' " Tate recalled. "Which was a great idea in theory but proved to be nearly impossible. The three of us have really different kinds of touring schedules that are pretty involved. Getting us all in the same country at the same time was incredibly difficult, and the same city even harder."

Trinity brings its show to the Sellersville Theater on Sunday. Expect a set that includes a mix of songs from each artist's catalog.

"This will be real fun," Tate said of the short tour, on which Sellersville is the third of at least eight stops. "Ripper and Blaze are guys I've known for years. We met through various tours that I've been on and they've been on. . . . I've already really admired their musicality and what they've done."

One difficulty for the trio is that they are all lead singers who have to sing harmony on one another's songs, Tate said. "It's really challenging to do harmony, especially when you've been lead singing for a long time - you get locked into the melody."

"It's a focus thing; you really have to hear that note in order to sing it. So, yeah, it's challenging to do and to do well, especially when you have loud, distorted guitars . . . which is part of the excitement of what metal music's all about - the distorted guitar."

For now, Trinity is focused on touring for much of the rest of this year. All three will be so busy afterward, Tate noted, that they'll have to wait until 2017 to reconvene.

Is there a record in the works? First, Tate has an acoustic, VH1 Storytellers-type tour in Europe, then he'll put the finishing touches on Operation: Mindcrime's final piece of the trilogy. Then, "we'll talk perhaps about doing a record, if everybody's still into it. We'll see."

Trinity, with Geoff Tate, Tim "Ripper" Owens, and Blaze Bayley: 7:30 p.m. Sunday at the Sellersville Theater, 24 W. Temple Ave., Sellersville. Tickets: $35, $50. Information: 215-257-5805, ST94.com.

@DebWNJ