We have found that TPAs have differing interpretations regarding which divisor to use under the new 401(a)(9) regs. Some say you use the divisor from the MDIB table based only on the participant's age if the beneficiary is not more than 10 years younger (beneficiary in this case is three years older). Some others say you should take the divisor from the Sec. 72 joint annuity tables based on the beneficiary's age and the participant's age less 10 years. Who is right? Any clarification on this would be greatly appreciated.

see question Q-4.
it says to use the table EXCEPT as provided in paragraph b

paragraph b is if spouse is sole beneficiary, in which case you use the longer distribution period between the new table or the joint and life expectancy.