Run a three phase induction motor on no load using split resistance starting technique using Line and Neutral of single phase supply to any two terminals of induction motor, connect third terminal with rheostat 100 ohm, 5A with a switch. Once power to ON the motor starts running switch off the rheostat.
Now the voltages three phase appearing on terminals of induction motor running on no load are having phase unbalance as well as voltage unbalance.
The machine is known as Arno converter, were common in electric locomotive to supply auxiliary motors on board the engine. Currently these are being replace by solid state single phase to three phase converters.
There are several ways to generate unbalanced voltage conditions. Some low-cost ways to create balanced or unbalanced voltage sags for experimental tests are well stated and explained in this paper (http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6389164&tag=1).
It depends on which condition you want to generate, the cheapest way is to connect a load (for instead a resistor or a light bulb) in series with one of the load phases, this phase will get a voltage drop, but it will depend on the load current. If you want to have an unbalanced voltage regardless on the load (at least almost) you can use a transformer in one of the phases, if you need to control the amplitude you may use a variac on that phase, the unbalanced will be only on the amplitude (in phase), but you may also build a phase shifter with passive components (which phase depends on the load current) or with transformers.
Those bench marks produce an unbalanced voltage in steady state, if you want to produce a transient then it would be more difficult.
I assume you don´t have a programmable AC power supply (; - P).