There are more similarities than differences.

All arose at the end of World War I as direct challenges to democracy. They believed that ordinary people could not be trusted to govern themselves and make good choices. The best and most efficient way to rule, so they believed, was through an all-powerful leader who makes all the important decisions, and people who take orders from those above and pass them on to those below.

All three believed in heavy-handed government control over the economy. All three exploited working people and organized labor very harshly. (Even the Communists, who professed to be the anointed spokesman of the 'working class.') Workers were absolutely forbidden to strike, and forced into government-controlled unions that put the interests of the state first.

Also, ironically, Communists claimed to hate Fascists and Nazis, who in turn professed to hate Communists, even though they all did just about the same things. In a telling move, the Communists and Nazis formed an alliance in August 1939 to conquer and carve up Poland, and found the negotiations remarkably easy because the two parties had so much in common.

There were some differences, but they were less marked than the similarities. Communists believed in seizing private property and having the government run everything. Fascists and Nazis thought it more profitable to let the businesses continue, but to control them tightly. As a result, Communist principalities were generally a lot poorer than Fascist and Nazi lands.

Here are a couple of curious similarities: Pseudo-intellectualism and brutality.

All three liked to project the facade that they were run by intellectuals. Communists pointed to the turgid writings of Marx. Nazis trotted out a stable of creepy theoreticians and the frightful screeds of men like Streicher, the editor of their newspaper. However, Nazi pseudo-intellectualism drove out all their best scientists, men of the caliber of Einstein, which is a good thing because the loss of so many brilliant minds led directly to Hitler's defeat in World War II. In Russia, the science of genetics came to be dominated by a quack named Lysenko, who enjoyed the favor of Stalin. No one dared challenge Lysenko's junk science for fear of ending up in an Arctic concentration camp.

Behind the veneer of fake eggheads, all were savagely brutal. The Nazi death camps are well known. Mussolini constantly talked about war for this and the struggle for that. Stalin was so notorious for his coarseness that even his bloodthirsty predecessor Lenin, whose speeches are filled with references to beating people and killing "enemies," complained about him. Between the three ideologies, they account for as many as 150 million deaths in the Twentieth Century of people who need not have died.