I have no experience with this myself, but I believe that the total RMD can be satisfied with any combination of distributions from the three buckets as long as the designated beneficiaries buckets are the same of all of the bucket. (CFR 1.401(a)(9)-8 Q&A 2) The taxable amount of any particular distribution would be determined based on the bucket from which the distribution was made.
A Roth 401(k) distribution is coded differently than a distribution from a traditional 401(k) account, so distributions from these two accounts cannot be combined onto a single Form 1099-R.
It's not clear how this might change with the RMD requirement for Roth 401(k) accounts having been eliminated for 2024 and beyond by SECURE 2.0.
