Here is the math - hope it helps.
There are six 3kW, 208V heaters connected balanced between the phases, so there is a 6kW load between A-B, B-C and C-A. The load current is 6000W/208V=28.85A. The three load currents are 120? out of phase with each other, so lets say Iab=28.85<0, Ibc=28.85<-120, Ica=28.85<-240.
The current flowing on Bus A will be Ia=Iab-Ica (likewise, Ib=Ibc-Iab & Ic=Ica-Ibc.)
Converting: Iab=28.85<0 = 28.85 + j0
.................Ibc=28.85<-120 = -14.43-j24.98
.................Ica=28.85<-240 = -14.43+j24.98
So calculating for current on Bus A: Ia=Iab-Ica =>
.................Ia= (28.85 + j0) - (-14.43+j24.98)
.................Ia= 43.28 - j24.98
Converting back: Ia=50<-30
The current on Bus A (Ia) has a magnitude of 50 (and is 30? separated from Iab)