The three missiles flew more than 600 miles and landed nearly 250 miles inside Japan's air defense zone, South Korean officials said.
The White House quickly condemned the launches.
"Today's reckless launches by North Korea pose threats to civil aviation and maritime commerce in the region," the White House said in a statement. "North Korea's continued development of its UN-proscribed nuclear and ballistic missile programs threatens the United States; our allies, Japan and the Republic of Korea; and our partners in the region."
Pyongyang did not give any notice of the impending launch, Seoul's Joint Chiefs of Staff told Yonhap News Agency, adding that Monday's launch took place just days before North Korea is due to celebrate the anniversary of the founding of its government.
North Korea launched two short-range ballistic missiles in September 2014, ahead of the same anniversary, Yonhap said.
A Joint Chiefs of Staff statement described the launches as an "armed protest" meant to demonstrate North Korea's military capability on the occasion of the G-20 summit and days before the North Korean government's 68th anniversary, the Associated Press reported.
While North Korea frequently conducts missile launches, Monday's incident raised alarms as world leaders met in neighboring China. North Korea has also made recent strides in its its ballistic missile program.
In early August, a Rodong missile fired by North Korea traveled more than 600 miles, the longest-ever flight distance by that missile.
North Korea's most recent launch took place on Aug. 23, when it fired a ballistic missile from a submarine that flew about 310 miles, again the longest flight by North Korea to date for that type of weapon.
The U.N. Security Council in late August called North Korea's missile launches "grave violations" of a ban it signed on all ballistic missile activity.
In June, North Korea sent a Musudan missile more than 870 miles high in a test launch that outside analysts said showed progress in efforts to acquire the ability to strike U.S. forces in the region, the AP reported.
The United States has about 28,500 troops stationed in South Korea.
The Associated Press contributed to this report.