Unbalance current in capacitor bank
I recommend you use the simulator to empirically derive the equation you are looking for.
Start with all caps the same value, vary the value of one cap pair, and obtaining the corresponding Io.
I believe that the current will be proportional to the "mismatch" between the two caps (per phase), so you will be able to determine the constant of proportionality (k) and thereby get your equation (Io = kC).  Also, I believe the vector addition of the 3 phases will give you the total Io.