AnnC
A few things...
Firstly, things that are off are often not really off, but in standby, and still connected. Those things can still use data. Satellite TV receivers, especially those of DirecTV, are notorious for this, and they can wipe out your data in short order, as they use data for more than just On Demand, and again, even when they are "off".
Secondly, depending on your speed and depending on what's using the data, it's entirely possible to burn through 3GB in fifteen minutes, or even less time, much less an hour.
Thirdly, the only way to ensure that nothing can use data when you aren't actively using the service is to unplug the modem.
If you'd like help in determing what is utilizing so much of your data please start a new topic in the Tech Support section, which you can do through the following link...
https://community.hughesnet.com/t5/forums/postpage/board-id/TechSupport
In the meantime, the following two threads may be of help...
https://community.hughesnet.com/t5/Tech-Support/Understanding-data-usage-data-loss-and-connection-paths/m-p/78602#M55972
https://community.hughesnet.com/t5/Tech-Support/HughesNet-Data-Management-Tips-n-Tricks-How-To-Eliminate-Phantom/m-p/41581#M29776