Is replacement of the timing chain on the 3.5 litre engine a DIY job? The manual refers to an adapter for setting the tension, so I assume that I will need to buy one from Mitsubishi.

If its anything like the 3L in the 2nd gens you wont need any special tools.
Its also a belt, not a chain.

No it is not a DIY job unless yo have done belts before, no offense but the fact that you refer to a chain and not a belt would indicate this

It is vital that you use the correct tension on the tensioner as it is a 2 stage tensioner. Too loose and you will jump teeth, too tight and bye bye water pump, cam shafts and or snap belt

Keep in mind the cam shaft sticks a fair way out from the bearing to the pulley, thus if too tight can snap off, i have seen this happen once.

If you have done belts before, the tool is actually very easy to make, you need a small tension wrench, pref the one used for auto trans belt adjustment.

this link is for the second gen cars just to give you an idea what has to happen
4 cyl and V6 lower down in the posts

go from there

http://www.mitsubishiclubaustralia.com/forums/viewtopic.php?t=1679

Thanks everyone for the infomation, it is much appreciated.