The word "these" refers to Miriam, Shammai, and Ishbah. They were the sons Bithiah bore for her husband Mered.
# Bithiah
This is the name of a woman.
# Mered's Judahite wife
The Hebrew text says, "His Judahite wife," but most versions understand "his" to refer to Mered. This refers to a different wife of Mered, in addition to Bithiah.