Creating a conditional column with null value

Based on your description:
if column a is null, then column a should equal to column b
if column a and column b are both null, then column a should equal column c
all else should equal to column a

I'm guessing that you mean something like this. If not, I don't understand.
if column a is null, then RESULT should equal column b
if column a and column b are both null, then RESULT should equal column c
else RESULT should equal to column a

If I do understand, is this what you are looking for?

z=
IF(
	ISBLANK( [ColumnA] ),
	IF(
		ISBLANK( [ColumnB] ),
		[ColumnC],
		[ColumnB]
	),
	[ColumnA]
)

(If the difference is between NULLs and BLANKs, maybe PowerQuery would be the place to make the substitution.)