The most reliable way is to do it as a maneuver to apply an aspect on a defensive item. Since the item is defensive, you need not activate it in advance - only when you'd need to tag the aspect for the defense it would activate automatically. That way, no wasted uses.
And yes, you'd only need power 3 for it, leaving you with 5 frequency.
On the other hand, if you could get power 6, it could apply two aspects at once for a +4 bonus - but only 2/day.
Do note however that since items, even defensive items, can be activated with mental stress instead of using up uses/day, the two-aspect one might be more effective in short, brutal fights - but less effective in prolonged fights.