Phase shift oscillator: confused about phase shift per stage?
You're thinking of a phase shift amplifier with three cascaded RC filters, with all the R's and C's the same value.  The issue is that the stages load one another -- this is why the gain is \$\frac{1}{29}\$ instead of 0.65 or so, and why the shift seems wrong.
If you were to put a unity-gain buffer between each stage, then your math would work out -- but you'd be using up op-amps like mad.