The only country that borders China, Russia, and South Korea would be North Korea.
The landlocked nation you're describing is Belarus. East: Russia West: Poland South: Ukraine North: Lithuania Belarus fits perfectly within these borders.
Mexico borders the US on the south and Canada on the north.
Russia is the country that borders Finland on the east. Russia is the largest country in land mass in the world. It covers one eighth of the Earth.
Nigeria
Mexico.
That is Mongolia.
China and Russia border North Korea to the north while South Korea borders North Korea to the south. Russia's border with North Korea is on the far north east tip and is very small. China's border with North Korea is the largest of the North Korean borders.
Japan is an island nation without any borders.
Russia.
The landlocked country that borders China to the south and Russia to the north is Mongolia. The capital of Mongolia is Ulan Bator.
North Korea is the only nation to border both China and South Korea.