The other three ports are hidden underneath a plastic adhesive backed cover. The cover was added to the back of the unit to reduce confusion regarding whether the MB8600 is a Router or a Bridge Modem. It is a Bridge, with the ability to bond ports if your service provider allows it. To remove the cover, power down the MB8600, unplug any Ethernet cable plugged into the port as well as the coax connector and power cord. Lift the label with a fingernail or a paper clip, as shown below. The other three Ethernet ports are beneath that label.
Please note that not all service providers support port bonding. Be sure to check with your service provider before trying to connect additional Ethernet cables.