The deceased have been identified as Shivanna (50), his wife Nagarathna (45), and their daughter Rakshitha (21).
The incident occurred in Halekempayyanahundi village in T Narasipura taluk.
According to preliminary investigations, the family is believed to have consumed poison inside their home.
Wedding was scheduled for the next day
The tragedy unfolded hours before Rakshitha was set to marry, turning what was meant to be a celebratory occasion into a scene of grief.
NDTV, citing police sources, reported that the family had been under significant emotional stress in the days leading up to the wedding.
Man accused of harassing bride-to-be
Investigators are probing allegations against a local resident, Ullas Gowda, who is said to have wanted to marry Rakshitha.
According to police, after learning that her marriage had been fixed with another man, Gowda allegedly began circulating messages and photographs and making disparaging remarks about her to the prospective groom in an apparent attempt to derail the marriage.
Officials believe the alleged harassment may have contributed to the family's distress.
Death note names accused, say police
A purported suicide note recovered from the house reportedly holds Ullas Gowda responsible for the deaths.
Police, however, said the contents of the note are still being examined and have not yet been officially verified.
Investigators are also analysing digital evidence, including messages and electronic communications, to establish the sequence of events leading up to the deaths.
Probe underway
The T Narasipura Police Station has registered a case, and further investigation is underway.
Police officials said all angles, including the allegations of harassment and the contents of the note, are being thoroughly examined before any conclusions are drawn.