It's got a very good maker's name on it, but the answer depends what condition it's in.
If it's been allowed to drift 3 semitones flat over a long period time, most likely nobody has bothered much about looking after it. If it isn't reasonably well in tune with itself, personally I wouldn't even bother to try, because it certainly won't hold its pitch any better afterwards.
If there is corrosion on any metal parts, that will increase the risk of breaking something. Of course broken strings are replaceable, but if you want to turn it back into the "top-quality" instrument it once was, you might need to consider completely re-stringing it, or risk having the replacement strings sound different from the rest.
If it's 3 semitones flat, the string tension is only about 70% of what it is supposed to be. You certainly don't want to raise it by that much all at once - you might even crack the iron frame. "Rough-tuning" the whole instrument up a semitone at a time might be OK - though half a semitone at a time would be safer!
If you discover any seriously loose tuning pins on your first pass, either give up, or (if you insist on doing things the hard way) figure out how to fix that problem before you go any further raising the pitch.
Unlike violins, pianos don't "improve with age" - the soundboard inevitably changes shape gradually under the string tension and loses its original resonance.
Despite all these negative comments, one of my favorite pianos (owned by a friend) is a huge English "no-name" upright dating from the middle of the 19th century, with a wooden frame and not overstrung - but if I used it as my regular instrument it would be reduced to firewood within a few months.