1998 ocean yachts open fuel tank question

I recently purchased a 1998 Ocean Yachts open/express. I was told by the seller there are three fuel tanks totaling 680 gallons. I see both valves to port and starboard tanks are open. I need help understanding if the fuel drains from P/S tanks gravity feed to the center tank. I filled all three tanks then ran the boat for two days and after the center tank is 3/4 empty and the P/S tanks are both almost full. I'm not clear how to manage the fuel tanks. I don't want to drain the fuel inadvertently from the engines. Any help on how OY setup these boats back then would help. I'm told the open is on the same hull as the 48' convertible but haven't been able to confirm