Impedence implies regulations . Different impedence means different regulations i.e different output terminal voltages for same input voltage . When the 3 trfr.s with differnt impedence i.e different terminal volatges , there will be a circulating current bett the trfr.s that will reduce the outward power flow . If X MVA of power is fed to the individual trfr.s and circulating current owind to variation of terminal voltage calls for y MVA of power , then the total outward power flow will be X-Y MVA . Tht is why normally it is not advisable to put trfr.s with different % impedence in parallel . Morever , the circulating current is likely to increase the thermal loading and load loss of the trfr.s forming the loop through which the circulating current flows . In practice , there is always some difference in impedence of the transformers in parallel causing difference in power flow at input and output.