The volume is calculated by:

The three factors correspond to the edges of the box andmust be positive (or zero).

Therefore the smallest possible length of x is x = 0, but then the volume is zero too. Actually you leave the metal sheet untouched.

The largest possible length of x is x = 3, but then the width of the box is zero and consequently the volume is zero too. Actually you fold the sheet of metal once.

Thus the domain of V is