All three appeared to be short-range missiles, rather than the long-range types created to be able to strike the United States, and were launched from Kittaeryong on North Korea's east coast.
The U.S military initially reported that two of the missiles had failed but, according to its later assessment, one appears to have blown up nearly immediately while two flew about 250km (155 miles) in a north-easterly direction.
The North earlier threatened to shoot four intermediate-range ballistic missiles toward the USA territory of Guam.
South Korea's presidential office and military said North Korea fired "several" projectiles in what was presumed to be a test of its 300-millimetre rocket artillery system.
The launches were conducted as the annual US-South Korea joint military drills, known as "Ulchi Freedom Guardian", were being held in South Korea.
In April 2009, Pyongyang launched a long-range rocket which flew over Japan in what it said was an attempt to put a satellite into orbit, but which was seen by the US, Japan and South Korea as a disguised test of a Taepodong-2.
Neither Japan nor South Korea confirmed the U.S. military's description of the weapons fired by North Korea as "ballistic missiles".
Eye Damage From The Eclipse And Your Health Insurance
You can burn your retina, reports Time . "So, any sudden change of vision that stays and is permanent, come and get looked at". But in case that advice was forgotten, here's how to spot if you or a loved one got an unhealthy eyeful of the sun's rays.
"We will continue to work with our interagency partners on a more detailed assessment of this latest launch and we will provide a public update if warranted", command spokesman Cmdr.
The North American Aerospace Defense Command (NORAD) determined the launches from North Korea did not pose a threat to North America, he added.
The United States and its "puppets" in South Korea should "act with discretion if they want to evade the historic moment of death", the statement said.
The missiles reportedly landed in North Korean waters about 250km away.
"The reality vividly shows that the U.S. ambition for stifling [North Korea] remains unchanged no matter how much water may flow under the bridge and the puppet group's ambition for invading the north remains unchanged", the state-run Korean Central News Agency reported.
The North's missiles have alarmed Japan since a Taepodong-1 overflew its territory in 1998. "We do view it as a provocative act against the United States and our allies", Tillerson said in an interview on Fox News Sunday.
The rival Koreas recently saw their always testy relationship get worse after Mr Trump traded warlike threats.