How to design a 3-phase resistive load bank?

^{simulate this circuit – 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.