It should be

3×economy flex = 100% of miles flown = 786 × 3 = 2,358
PLUS cabin bonus = 393
PLUS status bonus = 393

Total of 3,144 one way.

Was the booking made post-registration? If not, you get the usual amount.

Otherwise, it's possible that:

1. MMB simply isn't counting the IB leg at all, in which case it is, sort of, correct;
2. MMB isn't applying the triple avios bonus;
3. Unlikely because I haven't seen it in the Ts & Cs, but somewhere in there it could be buried that the offer only applies to tickets that are 100% on BA metal/not booked with a TA etc.