I have changed spark plug and the coil pack what else would cause the misfire on cylinder 3

 

It would help if we knew what you are working on. What engine?
Depending on what the misfire code is telling you, could involve fuel injector , signal to injector, signal to coil, possible valve spring problem. Best to have computer checked for any codes, places like Autozone will do this for you at no charge. Once we have the info , then can move in to narrow down the problem.

 

It is a 5.4L engine misfire cylinder 0303

 

If that is all you rec'd for codes, you are going to have to take the tourist route and that is :
check for spark at that particular cyl
make sure power is available to the coil for that cyl
Make sure the grounding signal for that coil is coming from the PCM
I believe that engine has "Coil on plug" ign syst. , quickie switch coils with a cyl easy to get coil from another cyl and see if misfire moves.
Listen/feel for the injector for that cyl to be operating.Make sure the plug on connections are good and snug
One of the last things to check would be to remove the pass side valve cover and check the valve springs for that cyl looking for a broken valve spring either on the intake or exhaust valves. Hope you installed Motorcraft plugs and the correct ones for that year and engine.
Sometimes a plug can get damaged on installation might try switching plug with another good cyl also.