Sacrifice can mean to kill (as in the Biblical story) in a religious ceremony or for some other end. Sacrifice can also mean to give something up. Mary is sacrificed initially because she lives as a subservient wife, doting and dependent upon her husband. She has given up any other kind of life to be the homemaker. Granted, she seems to be happy, but because of the expectations of women in her society and/or the circumstances of her life, she has been sacrificed to this role in life. It could also be said that she sacrificed any other role in order to perform this one. She is further sacrificed by her husband when he leaves her, essentially "giving her and the baby up."
The sacrifice of Patrick is quite simply when Mary kills him. She sacrifices him (out of conscious or unconscious hate, spite, or necessity). After he is dead, she initially doesn't care what happens to her because she feels justified in sacrificing Patrick. At this point, the baby is in danger of being sacrificed (killed or given up). Mary thinks:
On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they kill them both -- mother and child? Did they wait until the baby was born? What did they do? Mary Maloney didn't know and she wasn't prepared to take a chance.
Mary didn't want to sacrifice the baby, so she devised a scheme to make sure she eliminated herself as a suspect. (One could make the case that the baby was sacrificed in the end anyway, in having to live with a homicidal mother, but that is a speculative assumption.)
One could say that Mary giving them the lamb was the same as a person offering up a lamb or some other symbolic object to God; in return the person (Mary, in this case) would get something: elimination as a suspect as the detectives unknowingly eat the evidence. However, this still makes the lamb the sacrifice.
To call the detectives themselves as sacrifices, one would have to say that they were given up by someone to someone or something, for some purpose. One could say that Mary sacrificed them by strategically having them eat the evidence. Thus, they became the last sacrifices because in eating the murder weapon, they unknowingly eliminate the evidence, and in the end they will have to give up (sacrifice) the case.