Too much water. Dragon trees, dracaena marginata, are notoriously sensitive to overly wet soil. The roots rot, and the plant dies. If the stems are soft and the leaves are droopy, the rot has probably got into the stem and leaf tissue, at which point the plant is a goner. If you cut or scrape the stem, you'll probably notice a very bad smell, also. If the stem doesn't smell bad, it may not be rotting, and you can try to save them in the same way as the other 2.
You can try to save the 2 plants that aren't soft. Take them out of the pot, gently rinse off as much of the soil as possible, and repot them into fresh soil. Use a
cactus mixture, and add more perlite, about 1/2 again as much as you have soil.
When you water a marginata, always test the soil to make sure it's sufficiently aerated before you pour in more water. Use a kebob skewer as if testing a cake. There should be only a few crumbs of soil sticking to the skewer, and when you run it between your fingers, you should feel only the barest trace of moisture. You need to test the soil all the way to the bottom of the pot, not just the top couple of inches. When it feels like this, you can water again, making sure to moisten the entire soil mass by watering until you get a run of.
If you want to get some more watering tips, you could check out
http://www.youtube.com/watch?v=W9fkseDb1Ys
