Include only amounts that were in IRAs on 12/31/2017, not amounts that were in 401(k)s on 12/31/2017. 401(k) A's 12/31/2017 balance would *not* be included on Form 8606. A 401(k) is not an IRA.
Your basis in nondeductible traditional IRA contributions did not change as a result of the rollover for 401(k) B. What changed as a result of the rollover is the amount of that basis that can be applied to your IRA distributions or Roth conversions from traditional IRAs in 2017. The increase in your IRA year-end balance due to the rollover from 401(k) B decreases the proportion of your IRAs that is basis, and distributions are nontaxable in proportion to the basis in your IRAs. Whatever amount of the basis on 2017 Form 8606 line 3 that isn't part of your 2017 IRA distributions carries forward on line 14 to be applied to IRA distributions in future years.
