He doesn't necessarily add 3g of lead at 12. Each racquet has a different amount of lead at the tip depending upon how much they need to have the same swingweight.
Unstrung weight is measured without strings and without an overgrip. Between heavy 15L gauge strings, an overgrip, and a dampener, pretty easy for weight increase to be between 20-25g. I have data from Prostock Tennis on that front.
My swingweight is an estimation based on his previous career numbers and how much I've seen SW change in strung frames (between 30-40pts). This number has also been corroborated by impacting tennis.
Also, check the link in my signature for a full list of Rafa specs through his career.