Q: my 5S keeping downloading every 3 hours and using all my data, even when it is on wifi, it uses the data.  how do I stop this?

I just bought two new 5S's in June 2014, I had a 4 before this.  Every 3 hours it does a download and is using a lot of data, on both phones.  Even when it is connected to wifi, the downloads are using the data and not the wifi.  Which then makes us go over our data plan.  I  have turned off everything I can see on the phone.  The only way to stop it is to turn off the cellar data. How can I stop the download?  It defeats the purpose of having a smart phone if the data has to turned off at all times.  Does anyone know how to fix this?