The PS3 won't go online with your modem connected directly- it isn't set up to detect online access via the USB port.
What you will have to do is set up your laptop to act as a "gateway" and connect the PS3 via normal ethernet cable to the port on the laptop. The "how" will vary depending on your OS. A Google search for ["internet connection sharing" +Ubuntu] (said example is if you are using Ubuntu on your laptop, minus the brackets) should get you the step-by-step directions.