Is it true that Othman was left unburied for three days and buried in a Jewish cemetery or a filthy place, and why did the Companions in Medina not support him?

The report that Uthman, may Allah be pleased with him, was left unburied on a refuse dump of Banu so-and-so for three days is invalid, both in its chain of narration (sanad) and its text (matn). The weakness of its chain of narration is due to the weakness of Abd al-Malik ibn al-Majishun and his transmitting the narration in an mursal (disconnected) manner. As for the invalidity of its text, it is evident in the fact that burying both believers and disbelievers is an obligation upon Muslims. How could it be conceivable that the Companions would leave the Commander of the Faithful unburied for three days?

Indeed, such narrations are fabrications by the Rafidah. However, the support of the Companions for Uthman is well-established, but Uthman, may Allah be pleased with him, himself insisted that they not fight in his defense.