A 48-year-old man, and a 30-year-old man were arrested today under section 41 of the Terrorism Act after officers executed a warrant at a second address in Newport, Wales at around 05.10hrs.
A 25-year-old man was arrested on Tuesday at approximately 19:08hrs at an address in Newport.
The arrest was carried out by officers from the Met's Counter Terrorism Command, supported by colleagues from Gwent Police and the Welsh Extremism and Counter Terorrism Unit (WECTU).
Two other men who were arrested on Saturday remain in police custody.
An 18-year-old teen male was arrested by police near the port area of Dover 07:50hrs.
A warrant for his further detention has been granted by Westminster Magistrates' Court until 23 September.
A 21-year-old man was arrested in Hounslow at 23:50hrs under section 41 of the Terrorism Act. A warrant for his further detention has been granted by Westminster Magistrates' Court until 21 September.
A search is taking place at a residential address in Stanwell, Surrey, in connection with this arrest.
London Mayor Sadiq Khan responded to the arrest, saying that "there will still be significant activity today and over the days ahead."
"London will never be intimidated by terrorism," he added. "We will always defeat those who seek to harm us and destroy our way of life."
The attack injured 29 people, mostly superficial burns — were considered serious or life-threatening.
Peter Crowley revealed he recieved minor burns and much of his hair had been singed off.
Prime Minister Theresa May subsequently announced that the nation's threat level for international terrorism had been increased to critical, its highest level again this year.
A BBC News presenter Sophie Raworth, who was at the scene, said she saw a woman on a stretcher with burns to her face and legs.
'The primary concern is that a man is still at large but there will be good CCTV images of that person' says terror expert.
Transport for London said it is ‘investigating an incident at Parsons Green’ and urged customers travelling between Wimbledon and Earls Court to use alternative routes.
A London Ambulance Service spokesman said: ‘We are on scene alongside the Metropolitan Police at Parsons Green.’
London Fire Brigade said they were called to the incident at 8.21am on Friday.
Anyone who has information that would assist detectives is urged to contact the hotline 0800 789 321; if it's an emergency always call 999.