Yes, you can. But you should be aware of the limitations.
First, you cannot connect the USB-modem directly into the Raspberry Pi. You need a 3G router. You can then plug in your Raspberry Pi into the router with a cable. We’ve had good experiences with Dovado’s 3G/4G routers. They’re easy to set up and offer features to keep the link alive.
Second, keep in mind that data plans can be very expensive. While we’ve put a great amount of effort into minimizing the data consumption by Screenly, the nature of HD video and image assets is that they are large. As a result, you should keep this in mind when you sign up for your mobile data plan.