The 3-port fuel tank pickups have 3/8", 5/16" and 1/4" hose nipples. The 3/8" is supply to the fuel pump, 5/16" is vent to vapor canister (each tank vent will tee together, before running to the canister via a single steel line), and the 1/4" is fuel return from the mechanical fuel pump (the return line helps to combat vapor lock in hot climates - used on 3/4 and 1-ton chassis). If the fuel pump doesn't have a 1/4" return nipple, you can cap the 1/4" nipples on the senders.
The selector valve you need, 3-port vs 6-port, will depend on whether you are using the 1/4" fuel return to the tanks. Only the supply lines route through a 3-port selector valve. Whereas, the supply lines and the return lines route through a 6-port valve. Running the return through the selector valve is important to prevent the tanks from transferring fuel between them and potentially overfilling.
Regarding the valve, the paired nipples (2 on a 3-port; 4 on a 6-port) that exit the same side of the valve connect to the tanks, the single nipple or double nipples (1 on a 3-port; 2 on a 6-port) that exit the opposite side of the valve connect to the fuel pump as described above.
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