In the odyssey why did Odysseus have to go into the underworld, and who were the three mourners he met?

About the mourners, no clue. But, Odysseus came to the Underworld in order to meet the seer Tiresias and learn about the outcome of his wanderings. It was the witch circe who gave odysseus this task, and told him how to do it. She also sent the wind who carried Odysseus' ship to the farthest realm of Oce, allowing him and his crew to find the Grove of persephone that circe had indicated. Circe instructed Odysseus to go to a rock which is located in the place where the rivers "Pyriphlegethon" or Phlegethon and Cocytus flow into Acheron. There, Odysseus dug a pit around which he poured a libation to the dead, first with honey and milk, then with wine, and finally with water. Then, having sprinkled white barley over the libation, he invocated the spirits of the dead, and after the invocation he cut the throats of a ram and an ewe. Then the souls of the dead came gathering about the pit.