3rd ban

I wanna know what happens after I get a 3rd ban on my account

You know, if you quit breaking the rules you might not need to worry about getting banned...

Depends on what. If it's the same reason as first 2 I think you just get an increased duration.
For example, raid ninja looting is:
1st = 30 days
2nd = 60 days
3rd = 120 days
4th I haven't seen yet, possibly permanent
If it's for different things I don't think anything special happens.

Length of a ban varies from offense to offense and by the amount of times you did the same offense. Botting bans and staff disrespect are permanent for example while stuff like racism and the like is just a few days. It's unclear at what point you get permanently banned for the same offense.