How do I calculate the voltage in this circuit?
First of all, lets re-arrange the components to see what's actually happening. As you can see, you have a simple voltage divider:







simulate this circuit &ndash; Schematic created using CircuitLab

To get the voltage across A and B, use Ohm's Law:
$$
V = I*R
$$
To find the total R, you first need to find the parallel value of R1 and R2:
$$
R_p = {R1*R2\over R1+R2}
$$
and then
$$
R_t = R_p + R3
$$