Nested ISERROR?

Hi all

I don't think you can nest IFERROR, unfortunately, so here's what I'm trying to do:

A B C
1 54.6 #N/A #N/A
2 #N/A 44.1 #N/A
3 #N/A #N/A 78.9

IF there is a value in column A return that value, however, if there is an error, return the value in column B. Additionally if there are errors in both columns A AND B return the value in column C.

Any ideas?

SA