What is the correct calculation for voltage if the resistance is 3 ohms and the current is 4 amps?

What you are talking about is the electrical potential, or voltage, necessary to produce a current of 4 amps through a material with a resistance of 3 ohms. The typical way to solve such a problem is by using Ohm's Law, stating that the current produced is proportional to the potential across the resistor but inversely proportional to the resistance. In other Words, I = V / R, where I is the current in amps, V is the electrical potential in volts, and R is the resistance in ohms. Here we know R and I, so we rearrange this equation to get V by itself: V = I * R. So, to get the answer, multiply your 4-amp current by your 3-ohm resistance, and you will get 12 volts.