The keyboard ahold be USB which almost any computer has. The 3-prong cable? Can you give more of a description? If it's the one that goes to the wall (power cable), they do carry those for around $15.
The keyboard should have come works usb, and most computer or in this case Dell computer should have a usb port in the back. As for the computer 3 prong I'm assuming it's for the monitor you may need a div cable or something in line of that. Best way is to check online and see.
Thanks! Your answer is awaiting moderation.