OK so here's what happened with the triplet thing.
When jacks mum ovulated she released two eggs. Both of these eggs got fertilized by jacks dads sperm. One of these eggs underwent normal cell division and created jack. However, the other egg divided into two zygotes (balls of cells basically). Since the twins came from the same sperm and egg they share the same genetic makeup with the exception of the gender chromosome. But jack came from a different egg and sperm, so while he has his parents genes, he didn't necessarily inherit the same ones as the twins.
And that is how Alan and Amanda are identical twins but jack is only a fraternal twin.
And yes. I did get A*'s in biology.