Not quite sure what you mean here. I have a Blackberry and I can tether it to a PC as if it were a 3G mobile dongle and share my Blackberry internet with my PC. Or are you referring to when there is a very poor 3G signal and your Blackberry device (assume you are referring to your mobile phone) cannot access the internet? If the second, this article should explain how to set up an ad hoc network.
UK's best mobile network 2016: best 3G, best 4G, best mobile networks revealed