Using if and then to add values to a cell

Question

I want to be able to add a value from cell "a" to cell #1 only if cell #2 contains a value or text, then I want to add a second value from cell "b" to cell #1 only if cell #3 contains a value or text, finally, when cell #4 contains a value or text I want cell #1 to drop the values in cells "a" & "b" and add the value in cell "c". Can I do this using if and then?

thanks

Ian