PHB p. 165. It describes how to objects can be destroyed. It has a special entry for magic armor, shields and weapons. It reads: 'Each +1 of enhancement bonus adds 2 to the hardness of armor, a weapon or a shield and +10 to the item's hit points. For example, a +1 longsword has hardness 12 and 15 hit points, while a +3 heavy shield has hardness 16 and 50 hp.
This is (more or less) repeated on p. 217 of the DMG, but then comes along page 222 of the DMG, where it suddenly says: 'An attacker cannot damage a magic weapon that has an enhancement bonus unless his own weapon has as least as high an enhancement bonus as the weapon or shield struck. Each +1 enhancement bonus also adds 1 to the weapon's or shield's hardness and hit points.'
Obviously, the statement on p. 222 DMG contradicts the statements on p. 217 DMG and p. 165 PHB, when concerning hardness and hit points of magic armor/weapons. Now look at p. 184 of your 3.0 DMG, it says (word for word) the same as p. 222 of the 3.5 DMG.
Because of the conflicting statements, I can only assume that p. 222 of the 3.5 DMG was erroneously not updated to reflect the new take on hardness and hit points in the new edition. If you could not damage magic armor, shields and weapons without having a magic weapon yourself, they would have put it in p. 165 of the PHB. I cannot read it any other way.