The three note cell is the center of the piece. It forms the fondation for the seagull theme. As most of Seachanges is atonal (meaning it's not minor or major, or anything really) the 3 note cell proves some (if any) tonality to the piece.
The subtraction principle is completely different. It is with every playing of the melody, each time a note is reduced. It's first seen in the seagull theme. The first playing is 6 notes, when repeated however the note value is reduced to 5 notes. This contunes until there is only one note or nothing at all. 6,5,4,3,2,1.
Hope this helps!