All three appear to have been replaced by younger Kim loyalists, part of an ongoing transformation of the country's political and military establishment since the young leader took power in 2011.
South Korea's Yonhap news agency, citing an unnamed intelligence official, reported that defense chief Pak Yong Sik had been replaced by No Kwang Chol, while Ri Myong Su, chief of the Korean People's Army (KPA) general staff, had been replaced by his deputy, Ri Yong Gil.
Army general Kim Su Gil's replacement of Kim Jong Gak as director of the KPA's general political bureau was previously referenced in North Korean state media, and confirmed Monday by South Korea's Unification Ministry, which deals with North Korean affairs.
Shoring up power
Madden said the reshuffling at the top of the North Korean military was likely done for a number of reasons, among them preparation for the Kim-Trump talks and future South Korean negotiations and exchanges.
Political commissars under the bureau are stationed throughout the KPA and can influence the army's activity at all levels.
"(Kim) is not going to want these military commissars helping themselves to any of this assistance coming to the North," Madden said. "That was a problem during the sunshine period, a lot of misappropriation and malfeasance."
"From about June 2017 to earlier this year, the KPA general political bureau was under investigation by the (ruling Worker's Party), the first time in 20 years that the GPB was under investigation," Madden said.
By bringing the GPB firmly under the Party's and his own control, Kim is likely looking to avoid a repeat of the extreme action he had to take against his uncle, Jang Song Thaek, early into his rule.
Singapore summit
Trump met with former North Korean spy chief Kim Yong Chol for about 90 minutes Friday -- the highest-level North Korean official to visit the US in 18 years.
He stressed that the Singapore meeting is part of a "process" that will go on for some time: "I told them today, 'Take your time. We can go fast. We can go slowly.'"
South Korea's Blue House on Monday quashed suggestions a formal end to the Korean War could be announced at the Trump-Kim summit.
Kim is unlikely to spend much time in Singapore. Sanctions make it very difficult for senior North Korean officials to travel overseas, and Madden said Kim will also not wish to impose too much of a burden on the Singapore government, which will have to deal with onerous security arrangements caused by both leaders' presence in the city.
There has also been speculation Kim does not wish to leave North Korea for an extended period of time as this may encourage any remaining opponents to his rule to seize the opportunity to stage a coup.
Madden said any similar attempt to wrest power during the North Korean leader's absence was unlikely, pointing to Kim's previous trips to Beijing and Dalian in China, which went off without a hitch.
CNN's Yoonjung Seo contributed reporting from Seoul, South Korea.