How to design a 3-phase resistive load bank?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Delta transformer with delta load.


  Each step up transformer is capable of 800 VA [at 600 V].



From \$ P = \frac {V^2}{R} \$ we get \$ R = \frac {V^2}{P} = \frac {600^2}{800} = 450 \ \Omega \$.



  If possible, having some way to test with load imbalance would be helpful.


450 Ω is your minimum on any phase. You can use any load between 450 Ω to infinity (open-circuit) on each phase.