In summary, Vol 1 covers hard corals and clams in detail, Vol 2 covers soft corals in detail, and Vol 3 does not cover any corals, but rather all aspects of aquarium design, chemistry, equipment, general husbandry etc, etc.
If you have the money, then all three are worthwhile, IMHO. If you're short of bucks, you're right, Vol 1 is slightly out of date, and would be the one to drop. Vol 2 and 3 I would say are indispensible. Vol 1 is very good, but I'd guess that there are probably better, more up to date books on hard corals.
Q