Sum with 2 custom columns

Hi there,

So I'm trying to resolve something kind of simple, but I'm kinda new at metabase
I need to sum two columns that are already customized; these two have their values multiplied by 3, and I need to sum them up.

Mathematically speaking, it would look like this:

Value A - ([Value B * 3] + [Value C * 3])

I've tried doing them separately like
Column to Value B * 3
Column to Value C * 3

But then when I sum them up, it keeps bringing me the wrong value.