1996 F150 4.9l setting codes P0135 P0155 P0131 HELP!!

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Hello, For the last 4 years, these 3 codes (P0155 P0135 P0131)continue to come back. Truck is a 96 F150 4.9L 5spd. Over these 4 yrs I've replaced the coolant temp sensor and all (3) O2 sensors. All connectors and wiring appear to be in good condition. When I clear the codes, they come back quickly. Truck has always ran perfect with no driveability issues what so ever. It does, however, get horrible gas mileage - usually around 13mpg, which isnt very good for a light truck like this. Iam hoping someone can steer me in the right direction to fix it. Thanks very much.