three phase supply means RYB AND NEUTRAL 4 WIRE SYSTEM 

 ( phase to phase voltage is 415V and phase to neutral is 
230V )

Two phase supply RY OR YB and neutral

in 2 ph supplu there is no neutral , but in three ph supply 
one ph act as a neutral & all 3 phases are 120 degree 
spaced to each other.

2 phase supply ststem have 2 wire, one act as a phase and 
another act as neutral or we have for safty purpose we go 
to earth system so we add anoher one wire for 2 
phase.example:red,black,green wire.1.red -act as 
phase,2.black act as neutral,3.green act as earth.
for phase to neutral or phase to earth =240volts as per 
indian standard.
in 3 phase system has red,blue&yellow here also for safety 
need green wire used for earthing.RED TO YELLOW OR YELLOW 
TO BLUE OR BLUE TO YELLOW=415V.any one of the two phase we 
have 415v,but phase to earth we have 240v.
in the 3 phase system no need separate neutral wire because 
phase to phase have 120 deg phase shift,so by the cycle one 
phase alternatively act as neutral.

in polyphase altermating current two circuit used with 
voltage phase differing by 90 degree is called two phase 
supply system.
whereas atleast 3 conductor carrying electricl voltage 
phase differening by 120 degree to connect load is called 
three phase supply.