I want to know why indium and oxygen form In2O3, where does the 3 come from ?

Answer 1

The electron configuration for In (it's in group 13) is 2,8,18,18,3 (49 total electrons) and the outside 3 electrons are pulled off to give it a valence of +3, thus, In2O3. Just like Al. Here is the site at webelements.
http://www.chemicalelements.com/show/electronconfig.html