Choose some notes, at least 5. A good first choice is, for example, the D minor scale: D E F G A Bflat C D. However, you could try other scales, modes, or random collections of notes, etc.
Someone keep an even, moderate beat with a foot or the like. On each beat, each player chooses a note to play from the scale. Just like in the children’s game Rochambeau (rocks, paper, scissors), everyone makes a simultaneous choice of their note without knowing what note the others might play. Play Tee-Tee Notes until you are fluently playing melodies that harmonize and weave around one another.
Now add one variation. Player 1 plays only half or whole notes (2 or 4 beats long). This is called playing the SLOW part.
Player 2 plays the FAST part; eighth or sixteenth notes (2 or 4 notes per beat).
Player 3 plays the MEDIUM part, only quarter notes (right on the beat).
Now if player 1 doesn’t want to play the SLOW part anymore, he or she can be a thief and steal the FAST or MEDIUM part. Whoever has his or her part stolen, must immediately recognize that this has happened and accept the part that the thief was playing before.
For example, the FAST player could decide to steal SLOW. Then the SLOW player would have to give up being SLOW and would have to become FAST.
This game results in spontaneously improvised 17th century chamber music that is very pretty.