Using only whole numbers, how can you draw three rectangles with perimeter of 12 units

Perimeter means the sides add up to 12. 
Rectangles have two pairs of equal sides. 

2(one side) + 2(another side) = 12
2 (one side + another side) = 12 
(one side + another side) = 6 

To draw a rectangle with a perimeter of 12 units, the two adjacent sides must add up to 6. 
1 + 5
2 + 4 
3 + 3