Well I wouldn't use 3w defence against 3 strikers, as you have big holes in your defence. Look at it this way:

DR DC DL
STSTST

His central striker will keep your DC busy, while leaving his other strikers free to get in behind as your fullbacks are too far away. Also you have 3 defenders and 2 central midfielders against 3 St and 3 MC, so he has the numerical advantage there as well. I don't have the answer as to the best formation, as there are plenty I struggle with too, namely 43n3.