(1) We can come up with three packaging schemes: ① Put three boxes of candy left and right; (2) Put three boxes of candy before and after; 3 put three boxes of candy up and down;

(2) Three boxes of such candy are packed in one package, which saves the most paper, because the contact area of three boxes of candy is the largest, and the maximum surface area is 20× 15=300 (square centimeter), so four (20× 15) surfaces are missing;

(3) At this time, the wrapped candy has a length of 20cm, a width of 15cm and a height of 5x4 = 20cm.

Paper area required: (20×15+20× 20+15× 20) × 2,

=(300+400+300)×2,

= 1000×2,

=2000 (square centimeter);

A: It needs at least 2000 square centimeters of wrapping paper.

So the answer is: 3.