First of all I know the back three plugs are a pain. Replacing the front three are fine, except they will be more worn by the time you do the back three, which engine may not idle as smooth as you might expect.
Do you know the condition of the back plugs? If someone replaced the spark plugs previously, suppose they decided to do the same thing, that they will wait on replacing the back three plugs, but never got around to get it done.
I would recommend replacing all of the spark plugs along with new wires and applying a small amount of anti-seize to the spark plug threads. I do this for any vehicle that has dissimilar metal cylinder heads compared to the spark plug metal. It will help with removal the next time you replace the spark plugs again. If you do not know how or need help in replacing all of the spark plugs have a technician help you out.
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