Add a formula to a nested if function

Hello,

I have a fairly simple spreadsheet that works out CBM on inner cartons (when we have them).

Please see very basic attachment:
Column A - Inner ctn Width
Column B - Inner Ctn Depth
Column C - Inner Ctn height
Column D - Inner ctn Weight
Column E - inner ctn CBM - Formula =sum(a3*b3*c3)/1000000

I currently have an IF formula in columns B; C and D ( =if(a3="N/A","N/A","")
For Column E, I want a similar formula, but it has calculate if Cell A3 = N/A, then show N/A; If it is blank, then show nothing, But if it has a number in it, then calculate the CBM formula shown above (=sum(a3*b3*c3)/1000000).

This makes sense in my head, so I hope that I am making sense.

Any help that you can give on this, is most helpful.

THanks

J