3 phase 3 wire inverters = 3 * single phases + 3 * load terminals.
But in case of 3 phase 4 wire = , 3 * single phase with 4 wires including one as neutral.
In power distribution side three phase four wire system is more preferable,
yet if the load is industrial loading ,three phase three wire is used.
In three phase four wire system, with domestic unbalanced load,as unbalanced current get path through neutral, the phase/line voltage applied to appliances remains as per requirement, and hence the better performance is obtained.
As neutral dose not shift and remains at ground potential better voltage regulation is obtained even in fault condition, on any phase, and complete black out can be avoided
3 Phase 3 Wire Diagram :
3 Phase 4 Wire Diagram :
3 pahse 3 wire systems are also delta connections