If isnumber combined with then formula?

Hi All,
I am trying to do a nested If statement combined with a formula.
I have some invoices that bill me monthly and some quarterly. Monthly bills go in column B and Quarterly in column C. In column F I want to just know the Quarterly cost.
So essentially, if C is a number, then just return that number, all done. But if it's blank, then look into B and take that number and multiply it by 3 (months) to get a Quarterly total.

I tried something like this
=IF(AND(ISNUMBER(B3), ISNUMBER(C3)), B3, C3)

which returns the quarterly columns properly but then gives me zeros for the monthly, and I am aware I have not added in the *3 component either.

Any help appreciated!

Maybe getting closer with this idea...need help converting the IF FALSE piece into a working formula.)

=IF(ISNUMBER(C3), C3, "Multiply B3 by 3")
bloombergexample.jpg

Nevermind! Got it to work! SOLVED!

HTML Code: