If a box was moved through 3 displacements of unequal magnitude, how could the resultant displacement be zero?

Total displacement is the distance between the start-point and end-point,
regardless of what route was followed from start to finish.

If you want to see a total displacement of zero, resulting from
a lot of little displacements, all you have to do is make sure the
last little displacement brings you back home from wherever you
are.  Then, you end up where you started, and your total displacement
for the day is zero.

In three pieces:

-- Start.
-- Displacement #1 . . . . . Walk 3 miles east
-- Displacement #2 . . . . . Walk 4 miles north
-- Displacement #3 . . . . . Walk 5 miles in the direction 53.13° south of west
-- Total displacement for the day = Zero.  You're back where you started.