You will need offensive weapons and soldiers to attack a C3 country. One thing is don't forget to buy ammo. For example if you purchase 12 Destroyers then don't forget to purchase say 1200 Destroyer Torpedoes. Build up you army a bit first before you attack. There's plenty of time. Also read Ares C3 War Guide -it's somewhere here on the Forum I just forget where.
In attacking a C3 country you don't need to worry about defense as the C3 country will not attack you back -now another player that's different and you would need a good defense.
Concentrate largely on air power like precision bombers and attack fighters. Also have some land units so you can occupy parts of the country your attacking. Hope that helps.