Hitler, Mussolini and Stalin all claimed absolute power over the state. To control the populace and silence enemies they employed similar methods: complete control of the media, including censorship of any dissenting voices and the growth of a huge propaganda machine, a very violent police state used to quell the first sign of citizen dissent, and the cultivation of a cult of personality that elevated the dictator in question to godlike stature as all knowing, wise, and a good father to be instantly obeyed.
Since we don't currently live in a totalitarian police state, it can be hard to envision what it was like, but in all three states, neighbors were encouraged to spy on neighbors and report any sign of disloyalty. A heavily expanded police force could and did tap phones and spy on citizens. All newspapers and radio broadcasts were approved by the state. The writer Umberto Eco writes of his utter surprise, when, after Mussolini's fall, his mother sends Eco, a child, to buy newspapers and suddenly he sees more than one newspaper for sale: the idea of a diversity of pubic opinion had formerly been inconceivable to him. In Germany and the USSR especially, prisons were vastly expanded, torture common and purges of groups considered in opposition to the regime were carried out ruthlessly. In all three cases, the rights of the individual were subordinated to the needs of the state.
Stalin and Hitler differed ideologically, so Hitler favored industrialists, the rich and "great men," with an emphasis on men. He very much believed in hierarchy and loathed any kind of social leveling, so woman were placed in their "proper sphere" in the home, and men placed above them and everyone was expected to take his or her place based on their "race" at birth, with Aryans on top and Jews on the bottom. Stalin, in contrast, claimed to favor workers, and (at least superficially) promoted equality between men and women. Mussolini, as a fascist, was ideologically closer to Hitler, especially in terms of racial and gender hierarchy, and in looking back to the Greco-Roman world as a golden age, but was less ruthless in pursuing his agenda than either of the other figures.
That's a very large question to answer, so let me get you started. One similarity between the three is that they created vast police agencies that clamped down on dissent and civil liberties, maintaining the dictators' power, but they were also pitted against each other in competition which reduced potential challenges to their rule.
There were some key differences to consider. First, both Hitler and Stalin created mass prison camps to incarcerate opponents and "undesirables". Mussolini relied on thugs to reinforce his rule, but less on prison camps. Stalin also used mass deportations of ethnic minorities, and resettlement of ethnic Russians into areas of the empire where they weren't before, in order to solidify his control. Hitler did this also with Jews and Poles, but to a lesser extent.
Mussolini had no death camps, although Jews were still deported from Italy, especially later in the war. Hitler and Stalin used death camps extensively, collectively murdering in excess of 30 million people.