Police are investigating a report of men fighting in Edmund Road shortly after 7pm last night.
A 23-year-old man from Southsea was taken to Southampton General Hospital with a serious injury to his chest.
Officers attended and arrested a 22-year-old man from Kent and a 22-year-old man from London.
A 20-year-old man from Southsea was arrested later yesterday evening in connection with incident.
All three men remain in police custody.
A police corden was in place last night, cutting off access to Edmund Road between Fawcett Road and Francis Avenue.
A Hampshire Police spokesman said officers were called to a second incident just before 9.15pm in Quens Road, Fratton, after a report of an altercation between two men.
Both men were taken to hospital for treatment to minor injuries.
A 40-year-old man from Portsmouth and a 35-year-old man from Southsea have been arrested in connection with the incident and are currently in police custody.
Contrary to social media reports, police have said there is no evidence at this stage to connect the two incidents.
Portsmouth District Chief Inspector Jim Pegler said: ‘A number of officers and specialist units were involved in these two incidents in the city overnight.
‘Both are being investigated thoroughly today, at this time it doesn’t appear that they are linked.
‘I understand that residents will be concerned by what has happened, but there is no evidence that there is a wider threat to the community.
’Please do contact us if you have any information which could help the investigation teams.
‘The Neighbourhood policing team will be in the area today conducting additional patrols.’
Anyone with information should call 101, quoting 44170217412 for the incident in Edmund Road or 44170217591 for the Queens Road incident.