1. **State the problem:** We have a balance scale with weights on both sides. On the left side, there is one 3 kg weight and two carton boxes (each unknown weight). On the right side, there are three carton boxes. The scale is balanced, so the total weight on both sides is equal. 2. **Set up the equation:** Let the weight of one carton box be $x$ kg. Left side total weight: $3 + 2x$ Right side total weight: $3x$ Since the scale is balanced, these must be equal: $$3 + 2x = 3x$$ 3. **Solve for $x$:** Subtract $2x$ from both sides: $$3 + \cancel{2x} = 3x - \cancel{2x}$$ $$3 = x$$ 4. **Interpret the result:** The weight of one carton box is $3$ kg. **Final answer:** $3$ kg