The third use is the law as spiritual gift. It is like wanting to buy a gift for someone and finding out what they really like. It has nothing to do with averting God's wrath or earning salvation. It is the "love" motive in, "We should fear and love God so that..."
That's pretty much the way I understand third use. Although, as a side note, I recall being told by a person who has chosen to no longer post here that, with that kind of thinking, I had swallowed the Holy Spirit - feathers and all.
Although I understand "third use" as guidance/instruction in righteousness, I also acknowledge a difficulty with it. How do we deal with the reality that we are still sinners and, thus, unlikely to earn passing grades in the instruction. If we accept third use, we also have to confess a plethora of sins of omission and, when we do, the Law is functioning in "second use".