Regardless of being grandfathered in (allowed), you should update your drier to a 4-prong cord/outlet (which is ACTUALLY grounded). This does not always require

Regardless of being grandfathered in (allowed), you should update your drier to a 4-prong cord/outlet (which is ACTUALLY grounded). This does not always require stringing an entire new circuit — NEC (US) allows driers to be grounded with just a single 10awg wire. Particularly if you operate rentals, the legal liability is perhaps almost as much as NOT retrofitting GFCIs into baths/kitchens. SRC: not your electrician