3G is on an extremely high frequency, so the normal antenna tricks won't work.
Moving the antenna around, even just a little bit, can make a world of difference. If it's USB, buy a 5m USB extension lead (a few quid on Ebay) and try moving it around the room till you find a "sweet spot" where you can hang it on the wall or whatever.
If you have a "Three" mobile handset, use that as a "signal tracer" - move around the room with it till you find the most bars of signal. Then put the dongle there.