@Accountant-Man You obviously don't practice in a community-property state, where the living-trust paper mills churn out joint living trusts even when it's a mistake to combine separate property into them.  So every grantor trust I see, has two grantors.  I haven't seen any with three grantors, but I'm not in the part of the state where polygamy is still practiced.  

This question doesn't involve an investment account and nominee distributions, so that's a case for another day.  But there's no rule prohibiting a grantor trust from having an EIN.  And it's required, when the trustee is someone other than the grantor.  As when, the grantor is incapacitated.  I see that occasionally.  A 1041 is filed with nothing but a "grantor trust letter" in lieu of a K-1.  If there are three grantors, I don't see any problem with having three grantor trust letters.