BATE does the calculations using each time exactly the same full approach (each solution is treated as a mixture of acid and base of known Ca & Cb concentrations, we discussed it to the death in the past), so the answer is always accurate. There is no need (and it makes no sense) to use different models for each problem.

3.02 is an exact answer, found using exact numerical method. Not something you can do manually, but perfectly fit for computer model.

3 is an approximate answer, found using approximate methods - suitable for manual calculations. Apparently it is good enough.