You can, I've set my mates up so he uses his Optus 3G piece of crap thing. You need to plug your PS3 into your PC/Laptop via Ethernet cable and go from there. I've copied & pasted this from one of my previous answers.
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1) On your PC, open the network and sharing centre.
2) Under view your active networks, find the one that is your USB modem.
3) Next to "Connections:" it will say something in hyperlink blue, click it.
4) Down the bottom click "Properties".
5) A new window will open, click "Sharing" in the second tab at the top.
6) Check both boxes, "Allow other network users to connect through this..." and "Allow other network users to control or..."
7) Click settings and a new window will appear with about 10-15 check boxes, check them all. (Click OK with the new window that pops up after each one)
Close all 3 windows that are now open by clicking OK or close, please don't click cancel..
9) Turn the PS3 on.
10) Go into Settings > Network Settings > Internet Connection > Enabled
11) Go into Settings > Network Settings > Internet Connection Settings > Custom > Wired and follow the prompts through there.
12) Go into Settings > Network Settings > Internet Connection Test and let it scan through, it should be working now.
If it doesn't work then check that you checked those 10-15 boxes properly, if they are all checked then it's something to do with the PS3. Go back to step 11 and retry that.
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Depending on your operating system at times you might be asked for administrative rights when in the network and sharing centre, just click yes/allow if it does ask. Then just create a PSN account, it will ask you to update and away you go.
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