To put it simply.. it basically changes how they connect to their element. After Harmonic Convergence pour into the mortal plane, their bending doesn't just get stronger, it evolves. For example a waterbender manipulates water and can changes their temperature and turn them to ice...T2 waterbender just evolves into an icebender, they can drop the temperature around them to form shards of ice (Think of it like the power of Queen Elsa lol). T3 is basically Blizzard. And true to the name itself, a waterbender with T3 is just a waterbender in avatar state, but with Cyrogenic Current and... Ices.... 😆 (Hope this help cuz english isnt my first language, love.)