Its been thee weeks since my finch laid three eggs one hatched a week ago they other two haven't hatched?

Most likely they aren't fertile. Or the babies died in the shell before they could hatch. If you're oh-so-careful, you can aways candle your eggs a few days after the mom begins to brood. It works best in a semi-darkened room but you can hold a flashlight behind the egg and see the network of veins and the bird embryo in the center, in an egg that's fertile. The eggs also get darker as they get closer to hatching.