RMDs are calculated using the year-end balance of the year immediately prior to the year for which the RMD is being calculated.
(1) 401(k) A's RMD for 2017 should have been calculated using the 12/31/2016 balance. 401(k) A's RMD for 2018 is calculated using your 12/31/2017 balance.
(2) 401(k) B's RMD for 2017 should have been calculated using 401(k) B's balance on 12/31/2016. 401(k) B's RMD for 2017 was required to be distributed from 401(k) B before you rolled over any remaining amounts to the traditional IRA in 2017. 401(k) B has no RMD for 2018 because, if I understand correctly, it had a zero balance on 12/31/2017 because you rolled the entire balance in 401(k) B over to a traditional IRA in May 2017.
Your traditional IRA RMDs for 2017 should have been calculated on the 12/31/2016 balances of those accounts. If the rollover from 401(k) B in May 2017 established a new traditional IRA account, that traditional IRA had no RMD for 2017 because it did not exist on 12/31/2016. Your RMDs for the traditional IRAs for 2018 are calculated using the traditional IRAs' 12/31/2017 balances.
