Three Canberra men are on trial in the ACT Supreme Court this week accused of trying to launch an attack on a sex offender.

At the opening of the trial on Tuesday, the court heard one of the men had a video on his phone of the sex offender molesting an underage girl.

The man allegedly told police he went to the house to bang on the door, show the offender the video and educate him.

One man said they only wanted to turn him in to authorities, the court heard.

But prosecutors contend the men committed a crime and went there armed with a knife and the intent to break in and cause the man harm.