combat expertise and fighting defensively

[Jurkal]

combat expertise is -1 attack +1 ac
fighting defensively is -4 attack +2 ac (+3) with 3 ranks of acrobatics

does combat expertise benefit from 3 ranks of acrobatics?

[kinevon]

No, but Combat Expertise, like Power Attack, scales with BAB.

BAB +3 or less: -1 to hit, +1 dodge bonus to AC
BAB +4 - +7: -2 to hit, +2 dodge bonus to AC
BAB +8 - +11: -3 to hit, +3 dodge bonus to AC
BAB +12 - +15: -4 to hit, +4 dodge bonus to AC
BAB +16 - +19: -5 to hit, +5 dodge bonus to AC
BAB +20: -6 to hit, +6 dodge bonus to AC

Fight defensive: -4 to hit, +2 dodge bonus to AC, +3 if you have 3 ranks of Acrobatics

Dodge bonuses from different sources stack, so they work together.

And there comes a point where you are actually better off fighting defensively with Combat Expertise than going into Total Defense.

Once the dodge bonus to AC from fighting defensively with Combat Expertise equals Total Defense, which is only +4, +6 with 3 ranks of Acrobatics, the benefits of continuing to threaten your opponent, and taking attacks, no matter how penalized, is better than standing like an unhittable lump.

[Dabbler]

To be honest, to be worthwhile Combat Expertise should have adjusted to +2 AC for -1 to hit, scaling with each four points of BAB, otherwise it's just a gateway to other feats.