DISCLAIMER: All things Harry Potter belong to J.K. Rowling.
-- Nonfatally Dispose of Your Little Darlings --
Bellatrix Lestrange's devotion to the Dark Lord was such that nothing could stop her from attending the Death Eater's meetings, not even the birth of her children. Rodolphus didn't stay home to take care of the children, either. Although he obviously cared about the poor kids more than Bella did, he just couldn't do a sissy thing like that. The guys would never let him live it down if he did. He was Bellatrix's husband, not her wife.
And so the triplets (carelessly named Rodolphus Junior, Blayze, and Corvus) spent a lot of time at Aunt Narcissa's house.
However, when the triplets were four years old and Narcissa was pregnant with Draco, Lucius complained to the Dark Lord about the Lestrange children constantly being abandoned at Malfoy Manor and how stressful it was for Narcissa when their own son was about to be born soon...
And the Dark Lord ordered Bellatrix to stay home and raise her children, as was her duty as a pureblood witch. A duty he'd allowed her to neglect for far too long already. He even went so far as to disconnect her Mark from the magical network, meaning that she would no longer be able to feel his call even thought the Mark still remained physically in place on her arm.
She quickly discovered that - just as she'd expected - she had absolutely zero maternal instinct and very little patience. It didn't help that Narcissa had raised the triplets to be spoiled little brats.
Within the first week of looking after them, Bellatrix found herself telling her children, "Behave yourselves or Mother is putting you up for adoption!"
And they learned a little too late that Mother always makes good on her threats.
-end-