That 3" is to much to trim off the door---so that's not an option---I will be simplest to get another door of the right size---is this an antique door or a more common one that can still be purchaced in a lumber yard?
if you want to expand the opening you will first need to determine if the wall the door is to be located in is a load bearing wall and proceed from there. The type of wall and the load placed on the wall from above will be the most important aspect.
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