How would a RN sign her name if she has a BSc a BSN and a MSN?

Proper Etiquette for signing ones name with title and or Degree is to only use the highest ranking position, title and or the highest degree you posses in each field. So if your name were Bobert Bobson, and you were an RN with the above listed degrees you could sign your name:
Bobert Bobson, RN
Bobert Bobson, MSN
Master Bobert Bobson, Nurse (or Mistress Bobert Bobson, Nurse for a lady)
Bobert Bobson, RN, MSN
There are others that I could list but the first two are the most common, with the third being quite archaic. Outside of academic circles, the designation of just RN is most common. The fourth in the listing is still within proper etiquette guidlines because one can have a masters in nursing and not have a nursing license and thus not be an RN (under which case you would be considered a GN or Graduated Nurse not a Registered Nurse)
As for the multiple degrees, the higher degree supersedes any and all lower degrees, unless they are in a different field, since Nursing is a Specific science, your BSN overrides the general BSc, and your MSN overrides your BSC. If this were not the case you would see many Doctorate holders with more initials behind their name than PhD. If However you had a Bachelors in Business Management, and a Masters in Nursing for example you could sign, "Bobert Bobson, RN, BBM, MSN" without breaching etiquette.