I've been gradually working through my 60 k service so I can tell you a little bit about what you're looking at. In order to get access to the rear three spark plugs you're going to have to get that upper intake manifold/plenum off. Here's some of the stuff you'll have to deal with one way or the other:

1) Removing the battery and the battery tray greatly facilitates access to what you're going to need to get to. Good time to clean and charge the battery.
2) Your air intake system needs to come off too. Good time to change the air filter.
3) Your EGR valve is going to have to be decoupled from the upper intake manifold. Good time to replace the EGR gasket.
4) The throttle assembly has to come off too. Yet another gasket to think about.
5) Note on bolts: There are FOUR stay bolts. One on the left front. left rear, middle rear, and right rear - right behind the throttle assembly. You can't see the last two so use the force. This part was definitely NOT in the Hayes manual and it was all I could do not to go monsoon gorilla and just rip that thing off when I couldn't find them.
6) Take real careful notes. You're looking at close to 40 bolts you're going to need to keep track off... I sketched the motor and taped the bolts to their locations on the drawing. Whatever system works for you is great but I definitely recommend a system of some kind.
7) Once the intake manifold is off you've got the upper intake manifold gasket. Replace that and COVER the holes. If you drop a spark plug in one of those you are in a world of hurt.
8) From here you need to make some decisions. Things that make sense replacing are the spark plugs, ignition wires, valve cover gaskets (if needed), distributor cap (if needed), distributor rotor (if needed), and PCV valve. My valve cover gaskets looked good but I did everything else because I never want to take that intake manifold off again.

Once all that's done you've probably done all that could and should be done with the intake off.

A few extra notes.

1) That intake is made of aluminum. Everyone warned me to use a torque wrench or risk stripping or damaging that thing.
2) Keep an eye on your throttle cable when reassembling... I accidently screwed it in with the EGR gasket. Ended up having to replace the gasket again.
3) Unless you have incredible manual dexterity you're going to drop stuff reaching some of those rear bolts. I used my little magnetic pickup thingie about a half dozen times and I still have a 10mm wrench I have been completely unable to get out. It's stuck between the firewall and a maze of pipes, tubes, etc. I really want to let the car keep it but it seems to be right above something important with lots of moving parts.

Good luck!