Marxism-Leninism is the ideology developed by the Communist Party well after Lenin's death (only becoming codified in the mid-30s, iirc). Leninism refers to the political philosophy of Vladimir Lenin himself, mostly roughly the establishment of the dictatorship of the proletariat by use of a vanguard party that will create a proletarian state that will immediately begin to wither. Trotskyism can also validly be called a Leninist ideology. Think of it like Christianity: Jesus had his ideas and teachings, then the Catholic Church established a lot of doctrine well after Jesus' death and claims to be the sole valid body that can be called the Church; yet there are other Christian traditions such as Protestantism that have just as valid a claim to the role of the Church and it's probably possible to analyze the teachings of Jesus and his historical material conditions to develop an ideology of Christianity completely outside of the traditions of any church body.

Another main difference between Trotskysim and Marxism-Leninism is the ML concept of Socialism in One Country, which Trotskyism emphatically rejects. Trotskyism, like the other tendencies, also heavily criticizes the bureaucratization of Marxist-Leninist states, particularly the Soviet Union during Stalin's reign and after.

Stalinism tends to be a pejorative term to refer to Marxist-Leninists as the ideology was developed under Stalin's rule and, if I understand correctly, largely articulated by him. Most, if not all, Marxist-Leninists claim that Stalinism is not a thing since it is not a distinct or coherent ideology and nothing more than a phrase used to insult MLs. I have seen a distinction between ML and Stalinism somewhere, however, that made a distinction between the political structure formulated by Stalin (Marxism-Leninism)--the single party state, democratic centralism, socialism in one country, state atheism, focus on industrialism, etc, etc--with the political culture and policies witnessed during Stalin's rule--Purges, cult of personality, secret police, etc. Don't know if that's an accepted view or not, I usually see MLs defend Stalin rather than whitewash him out of Marxism-Leninism, but it's one I've seen before and thought it might be worth mentioning.