1996 Isuzu Trooper 02 sensors

Engine light was on for some time. Vehicle running very poorly. Pepboys did check engine light pkg. Results:02sensor circuit low.(Bank 1, sensor2) Cyl.3 misfire.O2 sensor Heater circuit(bank1, sensor1)02 sensor Heater circuit(bank1, sensor2). They wanted to replace rear right, right front sensors.I changed plugs, air filter, fuel filter and only premium gas. Two plugs were coated in oil. One obviously the 3cyl. The car ran great and light was off for about a week. Now worse. Money is an issue. Do flowchart diagnostic or spend the time and money and replace the sensors?What do you think would be the most cost effective cure to keep her going. She has been a Trooper. Thank You and Take Care.