They are 3 different things.
I will be very concise.
Marxism is Marx's ideology which has been misused and often misquoted. He wrote his ideas in the Manifest of the Communist Party.
Many German politicians tried to cherry-pick Marx's works. They founded German Communism.
This German Communism will influence Russian Bolshevism, 20th century. It is based upon authoritarian suppression of private ownership.
Socialism was created in France, Germany and Italy. In the 19th century.
Sozialistische Partei Deutschlands and Partito Socialista Italiano were for the socialization of labor. That is, labor is to be defended by the State who will side with the workers and not with the Capitalists. Even the Pope in his Rerum Novarum applauded this ideology.
Socialism defends private ownership, and in fact it wants workers to own their property.
So Socialism is light years away from Communism.
Russian Bolshevism is very anti-Christian.
And in fact they persecuted Christians in Russia.