While mating, I'd like put an equal space in between the three rectangles.
Do you mean you want them wrapping around the cylinder 120 degrees apart? If so you can do an angle mate between a plane on each rectangle and set it to 120 degrees.
Or do you want them equally spaced axially along the cylinder? If the first and last rectangles are supposed to be located ON the ends of the cylinder, then you can mate those first and then do a symmetry mate between the center plane on the center rectangle and a face on the other 2 rectangles. If you want the same gap between rectangles and from the ends of the cylinder, you can make a sketch with equal relations in the assembly and mate to the sketch.
As Jamil indicated, it's going to be difficult to answer your question without more information. In addition to the options he mentioned, inserting and mating one part, then creating the others with a circular pattern might work, depending on a number of things.
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