You Mean 7 AEB with +/- 3EV. 
Does that mean a 21 stop dynamic range?
More like 18. Specifically. -9, -6, -3, 0, +3, +6, +9
but in 7 frames not 1.
I guess you could also use EC to expand this range to +/-5 or 28 stops in total, in 9 or 11 frames shot in 2 steps
Step 1: +/- 3 EV and -5 EC 7 AEB yields: -14, -11, -8, -5, -2, +1, +4 and then Step 2: setting the EC to +5 yields the following 7 AEB's -4, -1, +2, +5, +8, +11, +14
I just realized I have never tried this on my 5D3... so this is just in "theory".
