By Kim Palmer

AKRON, Ohio (Reuters) - Three self-described anarchists pleaded guilty on Wednesday in an Ohio federal court to plotting to blow up a four-lane highway bridge near Cleveland in April, authorities said.

Douglas Wright, 26, Brandon Baxter, 20, and Connor Stevens, 20, all pleaded guilty to conspiracy to use a weapon of mass destruction, attempted use of a weapon of mass destruction and attempted use of an explosive device to destroy property used in interstate commerce, authorities said in a statement.

The three guilty pleas came before U.S. District Judge David Dowd Jr., who expects to sentence the men in November. All three had been scheduled to face trial on September 17.

Five men in all were accused of plotting to blow up a bridge 30 miles south of Cleveland that runs through Cuyahoga Valley National Park.

A fourth suspect, Anthony Hayne, 35, pleaded guilty in July to the attempted attack and agreed to testify against the others. The fifth suspect, Joshua Stafford, is undergoing competency testing.

In July, Hayne said he understood his plea meant that he would face more than 15 years in prison and possible probation for life.

On Wednesday, lawyers for Wright, Baxter and Stevens argued that their clients should not face enhanced sentences on the charges. The so-called terrorism enhancement calls for a minimum sentence of 30 years in prison and up to life.

Without the enhancement penalty, the three men face a minimum of five years in prison.

An undercover FBI agent sold the men inoperable detonators and plastic explosives and authorities arrested them on April 30 after determining that they planned to proceed with the attack.

According to an FBI affidavit, authorities paid a criminal informant more than $5,000 as part of their investigation into the group that began in October 2011 when he met the five men at an anti-Wall Street Occupy Cleveland rally.

U.S. Attorney Steven Dettelbach told reporters after the hearings on Wednesday that his office was not spying on protest groups such as Occupy and did not entrap the five men.

"Make no mistake; it was their plot," Dettelbach said.

Defense attorneys had questioned the role of the paid informant and plan to raise those questions again at sentencing.

The Ohio FBI undercover operation was one of a number of stings by federal authorities in recent years aimed at preventing attacks by foreign and domestic militants.

The FBI has said the men had no ties to foreign militant groups, and court documents showed that they considered several possible plots, starting with smoke grenades and then moving up to explosives, prosecutors said.

The case is not the first in which undercover agents have been used to gather evidence of suspected plots. A Moroccan man pleaded guilty in June to attempting a suicide bombing of the U.S. Capitol building in Washington in February. An undercover agent drove the suspect on the day of that planned attack.

Authorities also used undercover officers to gather evidence at the Chicago summit of the NATO military alliance in May. Three men described as anarchists were arrested then and accused of attempting to make Molotov cocktails to hurl at police.

(Reporting by Kim Palmer; Additional reporting by David Bailey in Minneapolis; Editing by Paul Thomasch, Steve Orlofsky and Lisa Shumaker)