So, with Fractional BAB, since class 'C' has full, basically the best I'm going to do is (14 x .75) + 6 = 10.5 + 6 = 16. So in this particular instance, not much different between Fractional and straight-up addition (not playing with level order).
As for Fractional Saves, is this right:
Fort: All 3 classes are bad, so regardless of how I level, it would be 20 x .33 = +6
Ref: Class 'A' is good, classes 'B' and 'C' are bad, so just use A's Ref (+12)
Will: Class 'A' is bad, classes 'B' and 'C' are both good, so it's 20 x 2/3 = +12, which ignores the bump from 1st level of 'C'.
In summary, using Fractional BAB & Saves:
At 20th level:
BAB: +16
F: +6
R: +12
W: +12
Correct?