You don't really need rectangles. This can be done with 3 stroked paths in Illustrator....
- Draw 1 path
- Apply a thick stroke
- Apply a stroke profile (On the Stroke Panel) so the stroke narrows at one end.
- Duplicate the path
- Change the stroke color
- Adjust the anchors slightly so they are offset a bit from the original
- Duplicate the path again for 3rd stroke
- Tweak 3rd stroke anchors and color
Really you just need to draw 1 path. You could use the black stroked path in your second image as a starting point.
Technically if one makes the base stroke super, super, thick, this could be accomplished with just 2 overlapping strokes....
Or .. slightly more advanced.... 1 path with multiple strokes using the Appearance Panel and the Width Tool to add some variation to the thinner stroke.
