Writing for Two and Three Parts
Two-Part Writing
In two-part writing the primary intervals used are 3rds and 6ths, along with some Perfect 8ves and 5ths. For dissonances or Non-Chord Tones, use 4ths, 2nds, and 7ths. Interplay between the voices and textural variation are important considerations.
Three-Part Writing
In three-part writing there is more flexibility. A third part can join the first two to complete sonorities, or it may provide a pedal point or ostinato.
The score below, Vive l’Amour, for Soprano, Alto, and Bass (SAB) stands as a complete two-part arrangement for the upper voices. The third part for Bass was added to the texture, creating more complete chords and extending the range. Examine the arrangement with close attention to the intervals between voices and the use of contrary, oblique, and similar motion. These are among the important choices you will make in crafting your own arrangement. 
Materials
Arrangement of Vive l’Amour for SAB a cappella choir (PDF)