Replacing the plugs should take no longer than 1 hr. remove boots and wires, make sure you have the proper size spark plug socket and extension. Do not over tighten, as they will brake off inside, snug it tight. And may want to replace the wires at this time also.
The OP stated he has a 3.o 6 cyl. 24 valve 3rd gen engine which requires the rear half of intake plenum to be removed to even access the back 3 spark plugs, reason just because it is open with all that has to be removed to even get there, esp. if you have the GTS as I do, you should change wires, timing belt, tensiioner/idler, and waterpump--if you ever have seen it you will agree it's actually cost effective because you will not be doing it again for another 60,000 miles, and all serpentine bests replaced, no matter, just because all that has to be removed anyway, hence less labor. Lastly, this is what is called an interferrance engine, which if timing belt would break or water pump seize up (waterpump acts like an idler for timing belt as well), the valves would get bent if that's all if you are lucky, because although a highly efficient engine, there's very little clearance between valve body and piston-cylinder head, whereas majority of engines are non-interferrance engine that uses a timing chain that actually needs extra clearance between aforementioned operating areas of combustion sequence. Timing belts are needed for interferrance (sp?) engine ]s and if you keep to exact maintenance schedule with this engine, using irridium NGK plugs, new wires of same quality, oil changes, long life coolant; you will only be spending this if you want to keep a really cool car and engine for 250,000+ years. It's not a Porsche nor is it a money-pit. PAY for qualified mechanic to do this and shop around on web/phone to different shops, avoiding the StealerShip/Dealer's Service Dept., unless you have deep pockets. Hope this clarified things and BTW the long life coolant is absolutely corrosion-free, lasting 100,000 miles and costs about few hundred bucks.