Police in Cardiff are investigating the attacks, in an area with a high student population, and said it was too early to know if they were connected.
Officers received the latest report of an attack on a woman, 19, at Gorsedd Gardens at 04:30 BST on Thursday.
It follows an incident at Cardiff Civic Centre early on Sunday, and a second in Cathays Terrace early on Tuesday.
A 40-year-old man from Riverside has been taken to Cardiff Bay police station. A 22-year-old, also from Cardiff, who was arrested earlier on Thursday, remains in custody.
However, officers stressed that it was too early to say whether the attacks are linked.
Chief Supt Belinda Davies told the press conference: "My message is one of zero tolerance towards anyone who commits these appalling crimes.
"Crimes like this are never the victim's fault, but I would like to take this opportunity to remind everyone to follow basic safety advice so they keep themselves and their friends safe."
The first attack on Sunday happened close to the main Cardiff University buildings in the civic centre of the city.
It came as thousands of students flooded back into Cardiff for the start of the new college year.
A Cardiff University spokesman said its security team had visited student residences during freshers' week to discuss personal safety.
"We're in consultation with our Students' Union and are discussing appropriate measures to help ensure our students' safety."
"We encourage anyone affected by these events to speak to someone in Student Support," he added.
Extra police patrols are also continuing in the area to offer reassurance to residents.
On Wednesday, South Wales Police also advised people in the area to walk in pairs at night, and to stay in well-lit areas.
A second man has been arrested after three sexual assaults in five days, all within a mile of each other.