I am skeptical about the spark plug diagnosis. 41K is too soon to have no less that 3 plugs fail. If they are merely going by three different P030X (X would be the number of the cylinder misfiring) code to condemn plugs, they are not doing their job. If they use a scope to measure primary and secondary voltages, then they'd know 100% that it's plugs
I hope you didnt have to pay for that diagnosis, because I am fairly sure it is wrong. This code means more than simply bad plugs.
Possible causes for P030X codes:
Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
There is also a TSB (technical service bulletin) out on 04 Aviators for vehicles with those misfire codes. Have the dealer check for:
TSB No.: 06513
NHTSA No.: 10019912
TSB Date: January 1, 2006
Engine And Engine Cooling:Engine
Description of Summary:
Engine misfire with diagnostic trouble code (dtc) pewee, p0302, p0303, p0304, p0305, p0306, p0307, p0308, and p0316.
If they wont, then go to another dealer.
Hope this helps