I have one central 300w power supply running at 5v. How can I split this into 3 equal "ports" that use 20amps each
The current rating on a power supply is a maximum. If you connect the 5V supply to a 5 ohm load, you won't have 60A through your load - you'll only have 1A.

The same thing applies for multiple loads - you don't need to worry about them "splitting" the 60A evenly, because they'll all take the current that they need. As long as the three devices need a total current less than 60A, you should be fine.