adocs,
3 hours would be the accumulated gain over the centuries of a clock ticking fixed seconds equal to 1/86400 of chazal’s days, over our clocks. The actual difference in the length of the day between chazal’s time and ours is only about 35 – 40 milliseconds!
If you are looking for a “practical” consequence lehalacha from this difference, consider the 18 minutes it takes unworked dough to become chametz. This is defined by chazal in maseches pesachim to be 1/40 of 12 hours or 1/80 of a full day. However, due to the changing length of the day, dough would become chametz on our clocks at about half a millisecond before the 18 minutes are up. You can see why I put the word practical in quotes!