magnetic stripe card door locks in a 3 story building.
A lot depends on how 'smart' the magnetic stripe card reader/door lock is. If it's just a 'dumb' stripe reader and a simple solenoid type lock assembly, then I think you would be better off with a minimum standalone type arduino board at each door. Perhaps communicating via daisy chained RS-485 type multi drop serial link shared with all the other locks and of course a master arduino. If wireless link is desired this could grow to a little complex and expensive. Power requirements also have to be known.
I think if you could provide a link to the specific door lock & strip reader you might get better and more specific recommendations.