Compare and contrast all three of Tom's owners in Uncle Tom's Cabin: how are they alike and how are they different?

All three of Tom's owners are selfish in that they put their own needs ahead of their slaves' needs, but aside from that they are strikingly different.

Tom's first owner, Mr. Shelby is a kind enough master and the slaves feel fortunate to be owned by him, and especially, to have his kind, Christian wife as a mistress. Tom has been allowed to marry and live with his own wife and young children in a cabin, hence the title of the book. The cabin means a tremendous amount to him, as it represents something akin to a normal life. However, in the distorted world of slavery his life is anything but normal, for Mr. Shelby has been careless and self-indulgent and gotten into debt that he could have avoided. As a result, to save the farm, he must sell Tom, as well as a four-year old slave. This is a cruel fate, but Tom accepts it to save the other slaves, for they would all have to be sold were the estate liquidated. We can understand Mr.Shelby as a person who will wreck the lives of slaves rather than make adjustments in his own comforts. He is not a terrible person, but definitely sees the slaves as existing for his own ease and convenience. He is the typical person who isn't deliberately cruel but also not one to sacrifice in any way for others. 

Tom's second owner, Mr. St. Clare, is also a kind man who indulges the slaves and allows them their humanity as far as the system of slavery will allow. He is more reflective than Mr. Shelby and hates the system of slavery, but he tolerates it as an evil he can't change. He does agree to free Tom, but dies before he can do so.

St. Clare's wife is a cruel, selfish woman, self-absorbed and completely lacking in empathy. She sells to Tom to his third owner, Simon Legree. Simon Legree, unlike the first two masters, is a sadist who treats his slaves entirely as property to be used up. He overworks them under the whips of overseers and keeps them in such poor conditions that few last long, but Legree feels it is more profitable to work them to death and buy new slaves than to care for them properly. He tries to dehumanize Tom and use him to abuse the other slaves, but Tom refuses. Eventually, because Tom won't capitulate to his ways, he beats him to death. 

Stowe's goal was to show that even under humane masters such as Shelby and St. Clare, life was precarious for the slaves, because of how powerless they were. They could be sold at any time and for any reason and ripped from their homes and families. In Legree, she depicts how bad it could be for slaves under a sadistic owner, as they had no rights and no way to protest the sadistic treatment.