I know FAN can vary in rates (mine is only 15%), but some have also said it varies in what it applies to. Previously the FAN applied to my Family plan core (meaning the $xx amount for the family plan with first line included in that price that showed on the bill) and also any iPhone data plans (my $30 unlimited and also each $25 2GB plan for the kids). It did not apply to feature phone lines. My understanding of the new Mobile Share plan is that the FAN will apply to the core data plan (for me, 15% off of the $120 10GB plan) but would not apply to any of the $30 cost of the 5 phones added onto the plan.
Also, you may not have meant it this way and I don't mean to be picky about semantics, but the way you've worded things is a bit confusing. You don't have 3 (10gb) lines in Mobile Share, but rather you have 3 lines sharing a 10GB data pool. Again, not trying to be picky, but just wanted to make sure we were on the same page.