Could an undead in 3.5e D&D such as a skeleton benefit from wearing magic items like rings of protection?
Undead can benefit from magic items like any other creature.
The only exceptions would be items that have conditions they don't meet or do things that don't apply..like CON bonuses. They could technically wear items boost their immunities but it would be a waste in most circumstances (poison/disease/etc).
Turn/Rebuke resistance would obviously be a great resistance to stack!
Caveat: for command activated items they must be able to activate and skeletons would generally be unable to activate voice triggers.