Always backup the original artwork before you destroy it by doing something irreversible. In this case, you may also need the original rectangles, if only to see that the silhoutte is correct.
You can work on a copy, moved away from the original so you can see and compare (you can just see the W and H values in the Transform panel, remembering that the Stroke Weight is to be added to stroked paths to get comparable values).
Presuming stroke/nofill paths, as far as I can see, the rectangles are not coinciding (unless they have stroke inside), so they are apart.
If this is the case, you will have to do it in multiple steps which can be done in different ways.
As one simple way you can, using the Direct Selection Tool and starting with nothing selected:
1) ShiftClickDrag the left side of the middle rectangle to the left right to snap to the right side of the leftmost rectangle (or past it so you have an overlap),
2) Repeat 1) for the rightmost rectangle,
3) Select all three rectangles and Pathfinder>Unite.
If you wish an actual silhouette as a filled path with the same outer measure, you can:
4) Select the path from 3) and Object>Path>Offset Path by half the Stroke Weight, then switch from Stroke to Fill, then delete the original path (now filled) from 3), it will be on top of the offset one).