For the equation \( y = 3x + b \) to represent a proportional relationship, it must be of the form \( y = kx \), where \( k \) is a constant. This means that there should be no constant term added to the variable \( x \).
In the equation \( y = 3x + b \), the presence of the constant \( b \) makes the equation non-proportional unless \( b = 0 \).
Thus, for the equation to be proportional, the value of \( b \) must be:
\[ b = 0 \]
When \( b = 0 \), the equation simplifies to \( y = 3x \), which is a proportional relationship.