Is it okay to breed parakeets with three birds in the cage?

The egg isnt DEAD just because its cold, they dont incubate an egg the second they have laid it. They wait for at least 3 or 4 more to come along. If the birds are not over 12 months do not add a nestbox and dont encourage breeding. Mating will happen without eggs usually unless there is a nest, but sometimes you will get the odd egg or two in a cage that means very little. If there is bedding or paper in the bottom of the cage being shredded up remove it. Make it as least nestlike as you can. Rearrange the cage so there is distraction.

IF they are old enough ( over 12 months and under 4 yrs for females..........and if you want them to breed, then add a nestbox. Dont put the egg in it, take the egg and keep it safe ( no need to warm it at all ) until the female lays the next egg in the box. Then add the first egg. Only do this IF they are old enough. AND if so remove the spare bird to another cage.