I have a 4.6L engine and I keep getting code p0303. And it's running real rough with white smoke coming out the tailpipe. Anyone who can help?

 

Read this.
https://www.obd-codes.com/p0303

Misfire on #3 could be spark plug, cop, fuel injector among other things. If white smoke is coming out the exhaust after the engine is warmed up, that is water vaporizing which may mean a head gasket failure. Perhaps a failure at the #3 cylinder. Check the compression on that cylinder.