In dogfights the main weapon is speed, especially now that everyone runs away when they meet someone with similar caliber pilots. SN and UK lack the speed to chase anyone down and force engagements. It used to be the same with KM but when they nerfed the attack they increased the speed abit putting it just between us/ijn and uk.
The upside to using T3s however is that most T4 players will think they have you at a disadvantage and think they can engage you easily. So if you are good enough with T3 fighters you can make them pay for underestimating you. SN's durability helps v T4.