what can be done to stop the spark plug form blowing out for the thrid
time

I believe ford sells a kit to to fix this. It's about $500 and comes with ten helicoils. This is a typical problem on the 99-2001 f150s with 4.6 and 5.4 engines. These plugs need to be changed when the engine is cold due to aluminum heads and the threads of the plug are steel. there is also only 3 threads for the plugs and the newer engine has five. Modmech and votermaniac seem to know their mod motors so I'd look close for their response

The first thing is to realize that this is a real issue, and that it is caused most often by EXCESSIVE PING (spark knock). If you have pinging, use a better grade of fuel.

Secondly, DO NOT allow anyone to replace your plugs unless they know how. This seems silly, but according to Ford, you MUST NOT use any oil, lubricant, or "antisieze" on the threads, NONE. You MUST also properly torque the new plugs to 13-15ft-lbs NO MORE, "snug", tight, and "good" are NOWHERE NEAR good enough, you MUST toq them properly. Of course, you must not cross thread them either.