Don't know if this was mentioned before. Before Envy, Sloth and Pride became gods, they were owned as slaves by Greed. Since they were slaves, rich folk in Greed's kingdom saw them as nothing more than pets to be owned and used to however the owner likes. They thought how lucky they were to be owned by someone 'as great' as Greed himself.

They were experimented on to test out injuries and medical equipment to see if they were right to use.

Sloth was experimented on by constantly breathing in toxic gases along with being injected by various poisons.

Envy was experimented on by being attacked by special animals to test their strength and aggression.

Pride was experimented on by being pumped full of drugs. Because of the drugs, he could barely move, barely talk and would momentarily go blind. Wasn't even able to think. He acted normal before the experiments, but the constant drugs ruined his mind.