The three women are on trial at Wolverhampton Magistrates Court this week for their part in an environmental protest at an Esso oil depot in Wood Lane, Birmingham in April last year.
Ruth Lanser, 26, Abigail Percy-Ratcliff, 23, and Hannah Torrence-Bright, 41, are on trial at Wolverhampton Magistrates' Court, accused of disrupting lawful activity by climbing onto a fuel tanker and sitting on top of it at about 5.10am on April 3 last year.
The court previously heard that the site went into a 'shutdown' state for much of the rest of the day, while a specialist police protest response unit was deployed to remove the protesters.
The defendant Ruth Lanser, a student, was called to the witness dock on Tuesday.
She said she first became involved with Just Stop Oil after attending a talk on the climate crisis.
She told District Judge Graham Wilkinson she soon learned about the protest planned on April 3, which she said was "well-advertised" ahead of time.
Asked by her defence representative Rosalind Burgin why she decided to climb on top of the tanker, she said she hadn't planned to do that in advance.
The objective of the protest, she said, had been "to stop the site from operating, but within that to try and send a message to the UK Government that it can't continue to fund oil".
When asked if she thought she was doing anything illegal, she said: "When I look at the climate crimes that our Government and Esso are committing, it's hard to see myself as committing a criminal act."
Prosecuting, Mark Fielding asked Lanser why the protesters didn't conduct their protest across the road from the Esso depot.
"If I had [done that] I wouldn't have been doing the protest that I wanted to do," she said.
"The reason behind it is much bigger than one site - it's to demand that the Government stops giving oil licences."
Mr Fielding put it to her that the protest was "nothing more than a publicity stunt" that did not result in any fewer oil licences being granted.
Lanser's fellow defendants, who are representing themselves in court, also briefly gave evidence.
The court was shown photos of the three defendants on top of the tanker. They accept being part of the protest that day with other individuals but have pleaded not guilty to obstructing or disrupting people engaged in lawful activity.
The court heard on Monday that a Sainsbury's fuel tanker was driving slowly on the site in the early hours of April 3, 2022, when protesters including the three defendants approached it and climbed on top of it.
Lanser, Percy-Ratcliff, both of no fixed abode, and Torrence-Bright, of Albert Avenue in Glasgow, deny obstructing or disrupting lawful activity and the trial continues.