The missiles were launched from the northeasten city of Hamhung, South Hamgyong province, into the East Sea at around 3.19pm (6.49am GMT) local time and covering a distance of 200km, according to the joint chiefs of staff of South Korea's armed forces.
"Our military is keeping close tabs on the situation and standing by with a heightened defence posture," South Korean officials told Yonhap news agency.
Pyongyang has yet to release an official comment on the tests, which come days after North Korea test-fired two ballistic missiles of its east coast on 18 March.
The missiles flew around 800km before falling into the water. The UN was quick to condemn the "unacceptable" tests and said the launches "constituted a clear violation of UN Security Council resolutions" in unanimous statement from the Security Council.
On March 17, US president Barack Obama issued an executive order imposing new sanctions on North Korea following a nuclear test carried out on 6 January and a nuclear test on 7 February, in which the country fired a long-range rocket that it claimed had put a satellite into orbit.