I see that "hence" means roughly "from this fact/time/place/source", while "thence" means roughly "from that fact/time/place/source".
Usage such as "half an hour hence" is typically (although perhaps not universally) used to refer to times in the future, being taken as "from this time" or "[forward] from now".
Would it thus be correct to say, e.g., "[today] is three years thence" — in the sense of "from that time" or "[forward] from then" — in order to allude to the day three years ago?