A London Ambulance Service spokesman said:
"We were called at 8.39am today to reports of a patient taken unwell at London Bridge station.
"We sent two ambulance crews, a single responder in a car, our hazardous area response team and a duty manager.
"Our staff have assessed four patients at the scene who were taken unwell. Three of the patients have been taken to hospital."
More top news
Scotland Yard confirm that two people arrested in connection with the attack on Westminster have been released with no further action
An appeal to support the family of murdered policeman Keith Palmer has reached more than £700,000 in two days.
PC Keith Palmer will be remembered permanently at the National Memorial Arboretum, Home Secretary Amber Rudd has said.