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Assuming you are working in ordered sheet metal.
Start by offseting each of the 3 base planes.
start each part as and assembly dirven piece of sheet metal.
The first sketch used to make the first tab of each part is created using includes from the planes. Flange are edited to me parallel to each plane.
Move a plane, the parts will resize.
Now back to your question about symmetrical
Off set two planes from each base plane, make one of them = to the other (negative) using variables.
If there's a lot of features to the left & right sides and you dont want to have to keep making the same steps on each or mirroring you cn alwasy simple just draw the left (or right) half and simply mirror copy the entire part about a reference plane.