Lately I've found out something about District 3 that's not in the wikia: District 3's Industry was factories in the 74th Hunger Games! I've added it to the page already, but here's my evidence:
The Hunger Games (book); Page 80
"For the opening ceremonies, you're supposed to wear something that suggests your district's principal industry. District 11, agriculture. District 4, fishing. District 3, factories..."