There are several things that can cause an egg to hatch early or late, heredity, humidity, how and how long it was stored before incubation (obviously not an issue for you), or just basic differences in the eggs. The really big one is temperature. If the average incubation temperature is a little warm they can be really early, if it is cool they can be late. I regularly have eggs hatch as much as two days early with broody hens as well as in my incubator but I don’t recall ever having one as much as three days early like you. You might want to confirm your temperature setting is right and that the incubator is not running really warm. Many thermometers are unreliable, whether they come with the incubator or just from the store. There are different ways to calibrate a thermometer, boiling water/ice water method is one but I compare mine to a medical thermometer that is already calibrated.
Often, even if they are two days early, all the eggs hatch in a relatively short time, say within 36 hours of the first one, sometimes even less. But sometimes the hatch stretches over three days, under a broody or in my incubator. If your incubator is running warm your hatch may not be as stretched out as you think. But it may be so you have to be prepared. It’s a good question.
Before they hatch a chick absorbs the yolk. It can live off of this yolk, food and water, for three or more days, I’ve had some hatch on a Monday and not eat or drink until Friday when I gave incubator chicks to a broody hen. But consider three full days as a reasonable limit. For some it is that short.
You often see warnings to not open your incubator during lockdown. It’s possible opening it when an egg has pipped can cause the membrane that surrounds the chick to dry out and shrink around the chick, keeping it from hatching. It doesn’t happen often but it does happen so I consider it good practice to not open the incubator during lockdown unless you have a specific problem to deal with. Why take an unnecessary risk? But there are plenty of people on this forum that regularly open the incubator during lockdown and just deal with the consequences if there are any. You can get some different opinions on this.
In my opinion, I’d wait for three days in an extended hatch to remove the chicks. That’s what I normally do unless I have an emergency to deal with.
