Engine turns over but will not start

Engine turns over but will not start. It began with having misfire codes on each cylinder. The van was running rough when idling so I go the recommended spark plugs and installed them. Turned on the engine still ran a little bit rough and a couple days later the check engine light came back on with code misfire cylinder three so I checked plug cleaned it and also cleaned the throttle body with JB cleaning lucubrate and started right up no problem. The next day I go to start the van and it has a hard time for a couple seconds it dose this for a few days and now it will not start. I check the plugs and they are getting spark, unplugged the gas line from the fire wall turned the key and gas came out for about three seconds. I also unplugged o2 sensor to check for a plugged catalytic converter and sprayed starter fluid in the throttle body. And it still will not start. Is it possible it could be a sensor or what could it be?