For my female vocal group, I wrote a three-part warming-up exercise. At first, all voices are singing the melody in unison once. Next, you may sing the exercise in three parts. The lower voice is singing the exact notes from the melody, but a fourth lowered. The high voices are singing in harmony with the melody a third above:
If you decide to repeat the exercise, a little higher for example or with different sounds, the idea is to start with the melody in unison again.
One of the advantages of this warming-up exercise is that the sopranos get to sing low notes as well (so they will train their low notes) and the altos get to sing high notes as well (to train their high notes).