Have you ever gone camping in the mountains and heated water over an open fire? If so, then you probably discovered that the water boils much more quickly at the higher altitude than at sea level, but at a lower temperature.
The one thing that determines the boiling point of water, no matter where you are, is air pressure. High up in the mountains, there is less air pressure pushing down on the water, so it boils at about 181°F., or 90°C.
Down at sea level, with more air pressing against the water, it must reach a temperature of 212°F., or 100°C before it will boil. Therefore, a 3-minute egg will cook in 3 minutes at sea level, but for it to reach that same degree of “doneness,” it will need more time in higher altitudes.