To evaluate the expression \( 3b - ac \) with the given values \( a = -2 \), \( b = 3 \), and \( c = 0.5 \), we can substitute the values into the expression:
\[ 3b - ac = 3(3) - (-2)(0.5) \]
Calculating \( 3(3) \):
\[ 3(3) = 9 \]
Next, calculate \( (-2)(0.5) \):
\[ (-2)(0.5) = -1 \]
Thus, we have:
\[ 3b - ac = 9 - (-1) \]
This simplifies to:
\[ 9 + 1 = 10 \]
Therefore, the value of the expression \( 3b - ac \) is \( \boxed{10} \).