In a 3 phase inverter what is the maximum voltage across each switch
Ideally the max voltage any of your switches could experience is equivalent to your DC source voltage (B2). In reality however, you would want your switches to be rated much higher then that. In real life inverters, the reasons to have higher rating for Vds are:


The rail voltage B2 could fluctuate when load rapidly changes. In many inverter designs like Variable frequency drives, the rail voltage maybe sourced from a boost converter maintaining a certain bus voltage. Transient spikes in voltages due to load fluctuations aren't entirely uncommon and could damaged your switches.
In real world, your load is not necessarily resistive, inductive loads would give your switches a hard time when you try to turn them off, especially when the load current is non zero. This could lead to rapid rise in Vds and I have seen devices frequently damaged due to this. 


For the reasons above, you would want your Vds maximum to higher then B2, preferably twice B2 and you would in addition want to put in additional protection to save your switches from breakdown. The choice of protection would vary and this could be taken care of using an efficient gate drive circuitry or other means(like fast TVS diodes).

Hope this helps.