An unnamed 25-year-old was held in a counter-terrorism raid in Newport, south Wales, shortly after 7pm local time on Tuesday. A spokesperson for the Metropolitan Police said a building in the city was also being searched.
Two other men, a 48-year-old and a 30-year-old, were arrested in Newport on Wednesday morning.
“This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday,”commander Dean Haydon, head of the Counter Terrorism Command, said.
“We now have three men in custody, and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack.”
Witnesses to the arrest described officers “jumping” a man who was put into a police car and driven away.
Searches continue at four addresses, police say.
Detectives have been granted more time to question the other two suspects.
An unnamed refugee, who is suspected of detonating the improvised explosive device (IED) on a train injuring 30 people last Friday, is believed to be an 18-year-old orphan from Iraq. He was arrested at the port of Dover on Saturday.
It is understood he had previously been referred to the government’s counter-radicalization program, Prevent.
A second suspect, Yahyah Farroukh, 21, from Syria, was arrested just hours later outside the chicken shop where he worked in Hounslow, west London.