Well it is different if you have the better team or not, but for the sake of the thread i will assume the teams are equal. I would say it would leave you too exposed, because it essentially leaves your defenders going 1 on 1 against strikers or attacking midfielders, and those are battles that defenders will lose. The rule of thumb I usually use (it came from a guide i read on here back when i started playing) is to have 2 defenders for each striker, and a Defensive midfielder or defender for attacking midfielders. Team mentality can also have an affect, so with a 3w defense i would avoid being attacking