The missiles were launched from the Hwangju region, said South Korea's Joint Chiefs of Staff (JCS) and flew about 500km (310 miles) to 600km.
The US said the first two were believed to be short-range Scuds while the third was a mid-range Rodong or Nodong.
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.
South Korea's Prime Minister Hwang Kyo-ahn said missile tests had been "a rare phenomenon in the past" but had been taking place "continuously" this year.
"The threat to our national security has grown very quickly in a short period of time," he said.
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.