Don't get insulted but I learned a long time ago to state the obvious. Did she hide a couple on you for a couple of days and you just noticed them yesterday? I did notice you said isolated.
What you are describing is not totally unheard of. Rare, but it happens. It takes about 25 hours for each egg to make its way through that factory and a hen normally times it so only one is in that system at a time. Sometimes that factory quality control system does not work just right. When that internal egg laying factory is out of kilter, she might release an extra yolk. If they are released about the same time, you can get a double yolker. If they are spaced out a bit, you can get two eggs the same day. Three too, but that is really rare.
What often happens is that the hen's shell gland does not have enough time to gather enough shell material for the second egg, so the second egg is often thin-shelled or even shell-less. When people get an occasional shell-less or very thin-shelled egg, this is often the cause instead of the hen not getting enough calcium. Her body has plenty of calcium for one egg a day, but her body cannot process enough for two eggs a day. If yours really did lay three in a day, I'd expect the later ones to be thin-shelled or shell-less.
