The show of solidarity comes as an inquest for the victims is to open seven days after Khalid Masood launched an 82-second rampage.
American Kurt Cochran, 54, retired window cleaner Leslie Rhodes, 75, and Aysha Frade, 44, died after the Muslim convert drove at pedestrians on Westminster Bridge last Wednesday.
The 52-year-old was shot dead by armed police after fatally knifing Pc Keith Palmer, 48, in the Palace of Westminster's cobbled forecourt.
A forensics tent on Westminster Bridge in London this morning
The Metropolitan Police Federation said the intention is to link hands across Westminster Bridge from the south to the north at the moment the attack began.
Organisers said the event will show "we will not be divided. Linking all nations, faiths, orientation and sexes".
It is expected to begin at 2.15pm, and a minute's silence to remember those who died will take place at 2.40pm.
The Ahmadiyya Muslim Youth Association will be involved, and around 500 participants are expected on the bridge, wearing T-shirts printed with the message "I am a Muslim, ask me anything".
The inquest for the victims will open and be adjourned at Westminster's Coroner's Court as police continue to investigate the attack.
A separate inquest for Masood will be opened at the same court on Thursday.
His widow Rohey Hydara and mother Janet Ajao have both spoken out to condemn the atrocity.
Mrs Ajao said she was "deeply shocked, saddened and numbed" by her son's actions and said she had "shed many tears" for his victims.
Ms Hydara said: "I express my condolences to the families of the victims that have died, and wish a speedy recovery to all the injured.
"I would like to request privacy for our family, especially the children, at this difficult time."
Scotland Yard has said there is "no evidence" Masood was linked to Islamic State or al Qaida but he "clearly" had an interest in jihad.
The force is continuing to urge anyone who was in contact with Masood on the day of the attack to come forward.
His communications on March 22 are a main line of inquiry, amid reports his phone connected with encrypted messaging service WhatsApp just before the attack.
Two men arrested in connection with the investigation remain in custody after police requested warrants for further detention.
Officers have until 6.50am on Thursday to question a 58-year-old arrested at an address in Birmingham on the day of the attack.
A 30-year-old man who was arrested in Birmingham on Sunday can be detained until 11.35am this Sunday.
On Monday the family of American tourist Mr Cochran said they bore no ill-will over the incident.
Mr Cochran and his wife Melissa, from Utah, were on the final day of a trip to London to celebrate their 25th wedding anniversary when they were mown down on Westminster Bridge by Masood's car.
Mr Cochran was killed and his 46-year-old wife was taken to hospital with a broken leg and rib and a cut head.
Ms Frade is believed to have been a married mother-of-two, while Mr Rhodes, from Clapham, south London, was described by neighbours as a "lovely man".
Twelve people are still being treated at hospitals across London.