When I played Hattrick I used mainly the 3-5-2 formation. Yes, if you want to win in Hattrick you should go for it. Invest most of the money you earn from your stadium with players that have good habilities in "inner mildfield". You can compensate your lack in investment in other areas by buying inner mildfields with good secondary habilities in "scoring" and defense.
One of the key factors to win a game in HT in to dominate the field, to see your players having the domain of the ball in the entire game (and the 3-5-2 is perfect for it). However, to face a team using the 3-5-2 I sometimes ordered the 3-4-3, with the intention of increasing my chances of a sucessful goal. Yes the midfield domain increased the score chances for the adersary team, but you always have some if your disadvantage isn't that big.
