The plus 3 to enfeebling is a constant effect. It doesn't increase your base by 3 points one time only, but whenever you are wearing it, it adds 3 to whatever your base enfeebling skill constantly while wearing it.
In other words, if you put it on when your enfeeb skill is 32, your spells would be cast with an enfeeb rating of 35. If your enfeeb skill was 72 when you put the earring on again, then it would raise it to 75 for casting for as long as you wear it, but you would lose the plus 3 effect when you took the earring off.