These are terms that I’ve used somewhat equivalently in everyday conversations to designate a non-democratic government, however I thought it useful to elucidate the differences.
Authoritarianism
This is just the umbrella term for any system which is centralized around a single authority. If all else fails, this is the general term for a form of government with concentrated power.
Dictatorship
This is a form of rule where power is focused into a single person or a group of people. Typically, the dictator has to maintain power by appeasing an inner circle of elites which facilitate his rule, e.g., military leaders and oligarchs.
Totalitarianism
By far, this is the most extreme form of authoritarian government. Totalitarian rule seeks to control everything, up to and including its subjects’ thoughts. Both the public and the private spheres are controlled by the government. The dictator is typically seen as charismatic, with a cult of personality around them. Mass surveillance, political repression, and every other tool is used to control. The USSR under Stalin is considered as such.