Gwoods
Firstly, Welcome to the Chandoo.org forums
Assuming
A1: Width
A2: Length
A3: Depth
Then use:
=IF(AND(A1="",A3=""),A2,IF(AND(A1<>"",A2<>"",A3=""),A1*A2,A1*A2*A3))
or
=IF(AND(A1="",A3=""),A2&" m.",IF(AND(A1<>"",A2<>"",A3=""),A1*A2&" sq m.",A1*A2*A3&" m^3"))