Introducing 2 new kittens to a 4month old kitten. Good idea or bad?

As long as they are all fixed, it should not matter if they are male or female....depends on their temperaments. Since they are all kittens, simply sit on the floor with the two new ones and let the 'old' one sniff. They will probably hiss and growl, but that does not necessarily mean they will fight, just discussing territory. If the 'old' one is open armed, let them play. Good luck with your new babies.