The 24-year-old had been in custody since Friday.
Investigators say they believe the killings happened Wednesday night, when neighbors overheard the couple fighting.
Police say that Ruiz spent a day in the apartment with the three corpses and then phoned his father and told him what he'd done.
When officers arrived at the house, they found a bloody scene.
The women and her children were dead of stab wounds. The children were aged 2 and 5. All three had been partly wrapped in plastic bags.
Authorities have not released the victims' names.
