In such a case, the total or effective resistance between terminals A and B is given by:
1/Rab = 1/R1 + 1/R2 + 1/R3
i.e. the reciprocal of the effective resistance is equal to sum of the reciprocals of the individual resistances.
If there are only two resistors, the above formula reduces to:
Rab = (R1*R2) / ( R1+ R2)
e.g., if two 1K resistors are connected in parallel, the effective resistance is (1*1)/(1+1)= 1/2 = 0.5K=500 ohm.
In Parallel circuit, the Voltage that pass through each resistor is the same.
V1 = V2 = V3
Reference: http://www.electronic-circuits-diagrams.com/tutorials/resistors.shtml