Prosecutors said Amiram Ben-Uliel chose the Dawabsheh family home and another house in Duma, on the assumption there were people inside and, before firebombing them, spray-painted "Revenge" and "Long Live King Messiah" on their walls.
The arson attack claimed the life of 18-month-old Ali Dawabsheh. His mother, Riham, and father, Saad, later died of their wounds. Ali's four-year-old brother Ahmad survived with burns on his body.
"This trial won't bring my family back," Ali's grandfather, Hussein Dawabsheh, said outside the courtroom in central Israel. "But I don't want another family to go through the trauma that I have.
Ben-Uliel's triple conviction carries a potential life sentence.
A second underaged defendant in the case entered a plea deal last year in which murder charges against him were reduced to conspiracy charges.
The indictment said that Ben-Uliel surveilled the Duma village with the unnamed minor, and the two agreed to attack and kill Palestinians inside their homes.
On July 31, 2015, Ben-Uliel went to meet the underaged defendant in a cave in the Jewish outpost of Yashuv Hadaat. The latter did not show up, and Ben-Uliel decided to carry on his plans alone.
He searched for a house that appeared to be inhabited. Then first threw a molotov cocktail through the window of a house whose inhabitants were not at home.
The convicted criminal then proceeded to Saad and Riham's house and threw the second burning Molotov cocktail through the bedroom window where the couple and their two children were sleeping, before escaping.
Ben-Uliel belonged to a movement known as the "hilltop youth," a leaderless group of extremist religious-nationalist young Jews who establish illegal outposts in the occupied Palestinian West Bank.
Investigators placed several suspects under "administrative detention", a measure usually reserved for Palestinians, that allows authorities to detain suspects for months without charge.
Yet, as the investigation into the attack dragged on for years, Palestinians denounced a double-standard, where suspected Palestinians are quickly prosecuted under a military legal system that leaves them with few rights, while Jewish Israelis are protected by the country's criminal laws.