If there are three of us - two males and one female - and time for prayer comes how should we form the line for prayer? Should the female be with one male in the same line or should the female pray behind the men alone? I have heard that no one should pray alone in a line, and that the person who does so should repeat that prayer.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The method by which two men and a woman could pray in congregation is as follows:

The two men would stand in one row, the person leading the prayer would be on the left side of the other person praying with him.  The woman would stand behind them, even if one of the men is a Mahram to her, or both of them.

This is what is known in the Sunnah.  In Saheeh Bukhari and Muslim, it was reported that Anas (Radiya Allahu Anhu) prayed behind the Prophet (Sallallahu Alaihi wa Sallam) with an orphan, and his mother prayed on her own behind them.

What is forbidden for one person to pray on his own behind a row applies to a woman if the line is constituted of women; she is not allowed to pray alone behind that line. The same is true for a man, he should not pray alone behind a row of men.

Allah knows best.