3 resistors, R1, R2 and R3 are attached in parallel across a 6V cell with resistances 3, 4 and 5 Ohms respectively. Calculate the current across each resistor.

First off draw the circuit. The voltage across each resistor is equal (i.e. 6V). From here we just have to use Ohm's law (V = I x R) to calculate the currents across each resistor. This gives us:
R1; 6V = I x 3 Ohms; I = 2A
R1; 6V = I x 4 Ohms; I = 1.5A
R1; 6V = I x 5 Ohms; I = 1.2A