It depends on how you want to connect to '3g' - you would need to install a software modem driver that works through a mobile phone, otherwise the only other way would be via a wifi hotspot, which defeats the point of your question.I'm not aware of a laptop card that allows dial up to 3g.
If your wireless goes down at home, why not just connect the pc to a modem on your broadband line?
Most modern phones will come with a utilities disk which include software to connect a pc via mobile to the internet.
I use the Sony Ericsson W850i. The software allows me to connect the phone to the PC via USB or Bluetooth, then dial up via GPRS or WAP - so there is a data transfer cost to my phone bill.
I cannot use it for 3g (You can't always get 3g coverage anyway).Perhaps someone out there knows a phone that has 3g dial up software for such an application.
The issue with phones is the transfer rate is slower than wifi, so unless you really can't find a 'hotspot' or connect to a line, phones provide a slower & more expensive connection.