Hey there,
1. A# is alien number. Not applicable if your spouse doesn't have one.
2. B18, not applicable for K3. Same for B17. See the instructions:
This form may be used to obtain a K-3 visa for your alienspouse. Fill out the form as directed, except assume that"fiancé" or "fiancé(e)" means "spouse." Answer QuestionsB.17 and B.18 by stating "N/A." Note that filing this formis only necessary to facilitate the entry of your spouse as anonimmigrant.3. If it doesn't fit the field, then fill in that field by hand (black ink).
4. You need to send in the NOA1 for the I-130 because for K3 petitions you need to show proof that you have an immigrant petition pending because it is assumed that after entry on a K3 you will adjust status (and I
think you need the I-130 at that point, but don't quote me on that part).
It's confusing at first, but familarise yourself with the guides and posts here and you'll get the hang of it soon.