Prioritized columns through If Statements, including blank cells

If at all possible, that'd be great if you guys could help me out with this one:

I need a 4th column to retrieve the next number, in case the first (priority) is missing. In this case, Cell Phone numbers are top priority. If it's missing (blank cell), then, retrieve the next one (business) and so on.

Three columns as below and result to show on a 4th column.

Residential | Business | Cell Phone

Any help will be greatly appreciated!

thanks in advance!