I would recommend your original hunch in the OP, to add spacers, though I would use chainring spacers rather than store bought washers (simply for the precision of getting the right diameter (internal and external). It is very common to need to add spacers when customizing your chainring set. It does not affect the performance in any negative way. It would only be a problem if somehow, even with spacers, it was still rubbing; or if the spacers somehow made the distance too great for your front derailleur to get to all three chainrings. I doubt of these would happen.