The third death was announced on Monday and five are critically ill in hospital, following the event in Haaksbergen in Overijssel province.
Video footage shows the modified pick-up with outsized tyres driving over a row of cars as part of the show, but then veering off course.
A boy, a man and a woman died.
Besides the five critically injured there are another 18 with lesser injuries, the Haaksbergen mayor's office said.
Helicopters helped ferry the injured to hospitals in three nearby towns.
"A so-called monster truck drove into a crowd of spectators. A terrible accident with two dead victims," town authorities said on their website on Sunday.
Video of the accident posted on social media showed the truck revving hard and then riding over six cars before suddenly heading towards spectators who were separated from the show by metal barriers.
Screaming, some managed to get away but others had no time to escape.
The driver of the truck has been detained and an inquiry has been opened, police said, quoted by local media.
Three people were killed when a "monster truck" ploughed into a crowd of onlookers at an annual motor show in the east of the Netherlands.