3G dongles usually needs drivers installed to use them, but you can't install drivers on the Chromebooks - however, you can, from other reviews, tether a mobile phone to a Chromebook, and use it's 3G data allowance, while out and about. Alternatively, you can buy a mobile wifi hotspot, known as MiFi, with a prepaid data plan, for the same purpose - try PC world, or maybe Argos for this.
The Chromebook won't support a 3D dongle but you can connect via your smartphone (Andriod). You can easily setup your smartphone as a wifi router - Settings/ Wireless and network settings/ tethering and portable hotspot - activate portable hotspot. Your chromebook wifi will connect to the internet via your smartphone. You could get your phone unlocked and insert a 3G broadband sim e.g. GiffGaff have good deals with no contracts.