Months before Lindsay Clancy strangled her three children, she lost significant weight, couldn’t sleep and complained of postpregnancy mental health issues, the family’s former nanny testified at her murder trial Wednesday. But the nanny never worried the Massachusetts mother might be a danger to her children or to herself. She was let go in December after Clancy, a former labor and delivery nurse, decided not to return to work. The defense sought to portray Clancy, 35, as a doting mother who acknowledged her mental health difficulties. Clancy wrote about her struggles to get newborn Callan to sleep, and constant worries of his being sick.