These definitions sometimes are used interchangeably, however, in order to communicate better, the terms have to be defined clearly in order to express the ideas better.
State. According to this source, a state is "a people permanently occupying a fixed territory bound together by common habits and custom into one body politic exercising, through the medium of an organized government, independent sovereignty and control over all persons and things within its boundaries, capable of making war and peace and of entering into international relations with other states"
Nation. This is an accepted definition for nation. "A people who share common customs, origins, history, and frequently language"
Government. Here is source for the meaning of government. "the governing body of persons in a state, community, etc."
In order to use the terms correctly we can say that State and Nation are very similar. From how I understand them, both are an aggregation of peoples, State puts more emphasis in a territorial boundary and Nation more on a cultural background. Government is the administrative body of the State or the Nation.