Obviously it is all three ways, but it is interesting that Michael says it both ways depending on whom he is speaking to. When he speaks to Carlo right before Carlo is killed he says "Ahhh that farce you played with my sister, did you think you could fool a Corlee-own? But when he is speaking Sicilian to Frankie Pentangeli, he says "anch'e tua nome e Corleone."
I also like how Senator Geary deliberately mispronounces it when he gives his speech and then says it with great Italian emphasis in Michael's office.