A mother of seven has pleaded guilty to murdering two of her children and killing a third by driving her car into a lake in Melbourne's west.

Akon Guode's three youngest children, 16-month-old Bol, and four-year-old twins Hanger and Madit died after she drove the car into Lake Gladman in Wyndham Vale on April 8, 2015.

Another daughter, six-year-old Aluel, was also in the car but survived.

Guode, 37, appeared in the Supreme Court on Monday and pleaded guilty to one count of infanticide, two counts of murder and one count of attempted murder.