Jan 4 (Reuters) - A federal judge on Wednesday gave three Republican presidential candidates permission to join Texas Governor Rick Perry's lawsuit seeking a spot on Virginia's primary election ballot.

Newt Gingrich, Jon Huntsman and Rick Santorum filed a motion to intervene in the complaint that Perry filed after failing to get the 10,000 verifiable signatures from Virginia voters required for inclusion in the March 6 primary.

Only Mitt Romney and Ron Paul managed to get on the Virginia ballot. Romney won the Iowa caucuses on Tuesday by a razor-thin margin over Santorum.

Like Perry, Gingrich, Huntsman and Santorum say the state's signature requirement violates the U.S. Constitution.

U.S. District Judge John Gibney Jr. granted the motion and ordered the defendants to respond to their complaint no later than Friday, according to court papers filed in the Eastern District of Virginia.

A representative for Virginia's attorney general, who is representing the board members, could not be reached for comment.

The judge's ruling was not a big surprise, said Lee Goodman, an attorney for defendant Patrick Mullins, who is chairman of the Republican Party of Virginia. During a Dec. 29 hearing, Gibney ordered the plaintiff to give notice to the other candidates so they would have an opportunity to join the case.

"We think intervention is appropriate and certainly did not oppose the intervention of the other candidates," Goodman said in a phone interview. "We think one global resolution of these legal issues is in the best interest of the candidates, the people of Virginia and the party."

Perry sued Charles Judd, Kimberly Bowers and Don Palmer, who are members of the Virginia Board of Elections, as well as Mullins. On Tuesday, Mullins' attorneys filed a motion to dismiss the complaint against him as the party chairman.

"It was the Republican Party of Virginia's desire for its primary ballot to feature all major Republican presidential candidates so that Virginia Republicans would have several choices," the memorandum in support of the motion said. "But that desire was tempered by Virginia's legal requirements, which the Chairman was duty bound to follow."

The next hearing in the case is scheduled for Jan. 13.

The case is Perry v. Judd, 3:11-00856, U.S. District Court, Eastern District of Virginia.

For Perry: Hugh Fain of Spotts Fain and James Nixon, Trey Trainer and Martin Beirne of Beirne, Maynard & Parsons.

For Gingrich: Christian Adams of the Election Law Center and Stefan Passantino, Randy Evans and Benjamin Keane of McKenna Long & Aldridge.

For Hunstman: Craig Engle of Arent Fox.

For Santorum: Cleta Mitchell of Foley & Lardner.

For Judd: Earle Getchell Jr. of the U.S. Attorney's Office.

(Reporting by Jennifer Golson)

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