The missiles were launched from the western city of Hwangju, said South Korea's Joint Chiefs of Staff.
The US said the first two were believed to be short-range Scud missiles while the third was presumed to be a mid-range Rodong.
It comes after the US and South Korea said they would deploy an anti-missile system to counter the North's threats.
North Korea is barred by UN sanctions from any test of nuclear or ballistic missile technology. But tensions have soared since it carried out its fourth nuclear test in January.
It has also conducted several launches in recent months, including a test of mid-range missiles in June which were considered its most successful yet.
Seoul has said activity detected recently in North Korea indicates it could be preparing to conduct a fifth nuclear test imminently.