Bank 1 Sensor 3 & Bank 2 Sensor 1

First sensor bank 1 sensor 3, I have never heard of sensor 2. Normally bank 1 is the drivers side and bank 2 would be the passenger side. Sensor one is the one that is before the catalytic converter and sensor 2 would be the one that is just after the converter or actually mounted to the back of the converter. Your van may have two converters per side and sensor three would be the one in the second converter. Before you change you oxygen sensors try and clean the connections with electronic parts cleaner. You may want to do the same for the tip of the sensor after you take it out. You can generally pick up the sensors at most auto parts stores for between 50 to 150 dollars each, just stay away from the universal sensors buy the OE connector style for ease of installation(simple unplug and replace the sensor then reconnect the connector and you are done.