My provider gives my 3GB of data and I don't know how to find out how much I have used or have left. Where can I find this information
3G Watchdog keeps separate totals for Wi-Fi and 3G data use, so there should be no confusion about whether it is from the mobile network allowance (3G data) or some wi-fi hotspot. The watchdog can also be set to disable your 3G connection at some level (eg 80%) before you hit your limit.
AVG (antivirus) now includes a data usage monitor, but just warns and does not disable, as far as I know. If I had a limit, I'd prefer to have the connection broken in case I started a high traffic application (eg world wide webcam) and might be charged for traffic above my allowance (network I used in 2012 charged around 100 GBP / US$ 150 per extra GB of data, something to avoid!).