Can I Boil A Single Egg?
Written by Rabbis Eli Gersten, Yaakov Luban and Moshe Zywica of the Orthodox Union
When boiling eggs in their shells, there is no way to tell in advance if the eggs contain a bloodspot. Therefore, there is a minhag (custom) to cook three eggs together, so that even if one of the had a bloodspot, that egg would be batel (cancelled) between the other two in the pot.
At the present time eggs are not fertilized, and even if they contain bloodspots, one need only remove that spot of blood, and the rest of the egg can be eaten. Nevertheless, many maintain the minhag to cook at least three eggs together.