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I was reading Harry Potter and the Goblet of Fire and in the first chapter I read that three Riddles were killed in the Riddle House.

Every version of the tale, however, started in the same place: fifty years before, at daybreak on a fine summer's morning, when the the Riddle House had still been well kept and impressive, and a maid had entered the drawing room to find all three Riddles dead.
_{Harry Potter and the Goblet of Fire, chapter 1: "The Riddle House"}

Who were these people, and how they were related to Voldemort?

Voldemort's father and grandparents.

Elderly Mr. and Mrs. Riddle had been rich, snobby, and rude, and their grown up son, Tom, had been, if anything, worse.
_{Harry Potter and the Goblet of Fire, chapter 1: "The Riddle House"}

Voldemort laughed again. Up and down he paced, looking all around him as he walked, and the snake continued to circle in the grass.

"You see that house upon the hillside, Potter? My father lived there. My mother, a witch who lived here in this village, fell in love with him. But he abandoned her when she told him what she was.... He didn't like magic, my father...

"He left her and returned to his Muggle parents before I was even born, Potter, and she died giving birth to me, leaving me to be raised in a Muggle village... but I vowed to find him... I revenged myself upon him, that fool who gave me his name... Tom Riddle...."

Still he paced, his red eyes darting from grave to grave.

"Listen to me, reliving family history...." he said quietly, "why, I am growing quite sentimental... But look, Harry! My true family returns..."

The air was suddenly full of the swishing of cloaks.
_{Harry Potter and the Goblet of Fire, chapter 33: "The Death Eaters"}

They were Voldemort's father and grandparents

It's explained in a following book that Voldemort killed his father and grandparents, whilst making it seem (to the Ministry) that it was his uncle Morfin who did the deed. (Emphasis mine)

“Meanwhile, in the village of Little Hangleton, a maid was running along the High Street,
screaming that there were three bodies lying in the drawing room of the big house: Tom Riddle
Senior and his mother and father.

“The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how
the Riddles died, for the Avada Kedavra curse does not usually leave any sign of damage…

[...]

“So Voldemort stole Morfin’s wand and used it?” said Harry, sitting up straight.

“That’s right,” said Dumbledore. “We have no memories to show us this, but I think we can be
fairly sure what happened. Voldemort Stupefied his uncle, took his wand, and proceeded across
the valley to ‘the big house over the way.’ There he murdered the Muggle man who had
abandoned his witch mother, and, for good measure, his Muggle grandparents, thus obliterating
the last of the unworthy Riddle line and revenging himself upon the father who never wanted
him. Then he returned to the Gaunt hovel, performed the complex bit of magic that would
implant a false memory in his uncle’s mind, laid Morfin’s wand beside its unconscious owner,
pocketed the ancient ring he wore, and departed.”

_{Harry Potter and the Half-Blood Prince, Chapter 17}