This is why I tell my clients past age 70 to consolidate their IRA's into one account.  How did they end up with three, and was it all in one place at one time?  What do you do if the account holder is still alive, and decides to take the entire distribution from the account that never had any basis?  Do you still file the 8606 and allocate some of the basis to it?  

Who is your client?  It doesn't really matter for the decedent's return, right?  So you're doing one/two/three of the kids' returns?  How are you allocating your fees among them?