If you do the math, the change in angle for a 3mm spread is miniscule. This is why it isn't absolutely necessary to resquare the dropouts parallel, though I prefer to do so (but then again I have the tools necessary to do this correctly).
Likewise with the RD hanger, if you have the tool go ahead and realign it, since it'll be off 4-6mm front to back over the diameter of the wheel (assuming it was right before). On the other hand, if it was OK before, then that error isn't enough to create any issues. In any case, if you have the tool needed to check and square a hanger it's always good practice to check it before mounting the RD, because you never know.
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