I don't know about the other two types, but TGA is opened by Irfanview. Irfanview is useful, free, downloadable software. It'll probably www.irfanview.com or something. Easily findable trough google. I (and others) have used this program to post screenshots on forums like these. (If you take a screenshot with F2 or F3 I don't recall which, it's saved as TGA as well.)

Good luck!