When two resistors R1 and R2 are connected in series to an emf source E of negligible internal resistance, the current following in the circuit is
\(I=\frac E{R_1+R_2}\)
When a third resistor R3 is connected parallel to the resistor R2, then, net resistance R1 + R2 || R3 decreases, and hence current through R1 increases.