Final answer:
The equation with a constant of proportionality equal to 3 is y = 3x. This equation shows a direct proportional relationship where for every increase in x, y increases by three times that amount.
Step-by-step explanation:
The equation with a constant of proportionality equal to 3 from the given options is y = 3x. In a directly proportional relationship, the pattern follows y = kx, where 'k' is the constant of proportionality and represents by how much y changes when x changes. In this case, k is explicitly equal to 3, making option C the correct answer.