The property of equality that gets us from Step 3 to Step 4 is the Addition Property.
In Step 3, you have the equation:
\[ 3x - 30 = 0 \]
To isolate \(3x\), you add 30 to both sides of the equation:
\[ 3x - 30 + 30 = 0 + 30 \]
This simplifies to:
\[ 3x = 30 \]
Therefore, the correct answer is B: Addition Property.