You will have a hard-time solving this problem by means of small signal analysis.
Instead of working with Bode plots, my recommendation would be to break down the problem into two nested control loops, and take an analytical approach to designing the controllers.
- Inner (three-phase) current loop
- Outer voltage loop
For this to work, you need sufficient output capacitance so that output voltage variations are slow with regards to the current regulation, and can be rejected by the current controller, possibly with a little "feedforward" assistance.
There is plenty of technical literature on how to design three-phase current controllers. One of my favorite papers is "Current and Flux Regulation in Field-Weakening Operation", by Fernando Briz, Alberto Diez, Michael W. Degner, Robert D. Lorenz.
Once you have a solid inner current loop, the outer voltage regulation becomes quite trivial.
Take a look at the PLECS "Boosted Motor Drive" demo model, which contains an example of a three-phase current controller as well as a DC voltage regulator. Not precisely what you are trying to do, but very similar building-blocks in terms of the controls.