LAS Vegas isn’t big enough for THREE country music su­perstars – at least if SHANIA TWAIN has anything to say about it!

The “You’re Still the One” singer, who pre­miered her Sin City stage show at Caesars Palace on Dec. 1, is furious that Tim McGraw and wife Faith Hill are stealing her thunder on the Vegas strip. Just six days after Shania’s show opened, the Nashville power couple, both 45, debuted THEIR new show just down the road at the Venetian hotel and casino.

Shania’s beside herself,” said a close source. “This was supposed to be her big comeback after eight years away from the music scene. But now Shania feels like Faith and Tim have eclipsed her moment in the spotlight – and she’s seething!”

Shania, 47, is scheduled to per­form 60 shows over the next two years at Vegas’ nearly 4,300-seat Colosseum. She last toured in 2003-2004 to promote her 2002 album, “Up!”

The country beauty went into seclusion in 2008 after learning that her then-husband, record producer Mutt Lange, was hav­ing an affair with her best friend, Marie-Anne Thiebaud. In the aftermath of their breakup, Shania suffered severe vocal problems that left her unable to sing.

But Shania persevered, marrying Frederic Thiebaud – Marie-Anne’s former spouse – and worked to get her voice back in shape.

“It’s taken Shania everything to get to this point,” said the source, “and now, along come Faith and Tim to spoil her party. She knew they were coming, but she didn’t think they’d try to steal the spotlight like they have.”

According to the source, Faith blasted Shania’s show for being too over-the-top and believes her more intimate “Soul2Soul” concerts with Tim – which will run on select Friday and Satur­day nights through April – will be a much better bet.

“But don’t count Shania out,” added the source. “She’s got a big, glittery show and the determination to become Las Vegas’ No. 1 ticket.”