Re: does line voltages (R,Y,B) remains the same in 3 phase system ?
In theory yes, in practice no. If you are ONLY using three phases, and are measuring the voltage between say R and Y you are probably safe to do as you propose but if it's a four wire system with a neutral wire as well ('Y' rather than delta configuration) and you are measuring R-N, Y-N or B-N you might have to take into consideration that different loads on the phases cause voltage differences due to wiring resistance and possibly inductance.
Brian.