Can anyone help with the following vloopup formula?

=IF(ISNA(VLOOKUP($A4,RANGEA,3,FALSE)),0,VLOOKUP($A 4,RANGEA,3,FALSE))+IF(ISNA(VLOOKUP($A4,RANGEB,3,FA LSE)),0,VLOOKUP($A4,RANGEB,3,FALSE))+IF(ISNA(VLOOK UP($A4,RANGEC,3,FALSE)),0,VLOOKUP($A4,RANGEC,3,FAL SE))

If there is a value in all three ranges it returns the currect amount.

Here are the problems:
- If RANGEA is missing a value it does not add RANGEB and RANGEC.
- It RANGEA and RANGEC are missing values it adds RANGEB to a value from
the cell above.

Any suggestions?

Thanks