Courtney Barker did not have a driving licence
Unqualified driver Courtney Barker, 23, was at the wheel when the car fell 30 feet into the Blue Lagoon Lake in Arlesey, Bedfordshire.
Witnesses told how they saw him drive toward the water holding the front door open with his right foot.
Imran Khan, aged six, Aaron Khan, five, and Sharla Blair, three, were trapped in the back seat and died in the incident last July.
Barker, a forklift truck driver from Luton, was convicted of three counts of manslaughter by a jury last month, following a trial at Luton Crown Court.
'Grossly irresponsible'
Passing sentence on Friday, Mr Justice Wright said Barker was an "essentially decent and caring person".
But he had been guilty of "grossly irresponsible behaviour" which fell "little short of insane".
There were gasps from Barker's family as Mr Justice Wright imposed three separate five-year prison sentences to run concurrently.
Barker wept as the judge told him: "I have to say that that conduct was some of the most grossly irresponsible behaviour that it is possible to imagine.
"To drive in such a manner up to the edge of a 15 foot cliff which had a drop of 30 feet into water is conduct, and I choose my words carefully, which is little short of insane."
The tragedy occurred on 28 July last year when Barker drove his girlfriend Claire Armstrong, 23, friend Ryan Purdy, 22, and the three children to the lake to swim and sunbathe in his Vauxhall Astra estate.
No licence
The three adults escaped unhurt from the plunge, but the children were trapped in the back seat and drowned.
During Barker's trial last month, the jury had heard eyewitness and expert reports that he had driven across the field towards the lake holding the driver's door open with his right foot.
But Barker said he had been using both feet to operate the pedals and claimed the accelerator pedal had jammed as he approached the cliff.
Jurors were also told that although he had been driving on the road for more than a year, Barker had never taken a driving lesson or passed a test.
He was not insured, his car had no tax and no MOT certificate.
Outside the court, Barker's solicitor, Alan Jones, said the sentence was "much longer" than the family had thought and that an appeal against the conviction and sentence was being considered.
"The family have asked me to say that nothing, sadly, can undo the tragic events which occurred in July last year or bring the children back," said Mr Jones.
'Distraught and upset'
"They are deeply grieved. Mr Barker and the family wish to try to rebuild their lives as far as possible.
"They are very distraught and upset by the sentence passed today and indeed by the fact that Mr Barker was found guilty of these offences.
"An appeal is very urgently going to be considered both as regards the conviction and the sentence passed today."
Mr Jones said that there was no doubt that Miss Armstrong would be standing by Barker.