to be able to use 3G for internet while connected to wifi?
You have a new option from iOS 9,

Settings -> Cellular -> Scroll to the bottom of the page -> Wifi-Assist


  Automatically use cellular data when WIFI connectivity is poor


You have some articles about that all around the web
http://appleinsider.com/articles/15/10/14/apple-posts-ios-9-wi-fi-assist-guide-in-wake-of-complaints-about-excessive-data-use
http://www.macworld.com/article/2987233/smartphones/how-to-turn-off-ios-9s-wi-fi-assist-to-save-your-data-plan.html