How can I do this in Excel with Multiple IF statements:-

The conditions:-
The IF function has the following structure:
IF(logical_test, [value_if_true], [value_if_false])
You can nest further IF's in the [value_if_true] and/or [value_if_false] arguments.
In your particular case, paste this in E2 and copy-drag down:
=IF(ISERROR(C2),IF(ISERROR(B2),D2,B2),C2)
Now, this assumes that your dates are sometimes #NULL! like your question suggests, but I doubt that this is what you actually mean. Maybe you mean blank? Or some other value? If you do mean blank, then replace ISERROR with ISBLANK in the above.