t +3 is a bit of a fallacy with some brokers particularly online banks.
You buy today and it must settle in t +3. Doesn't matter if you sell tomorrow because it won't settle in time. Not sure how tolerant a bank would be if you did it regularly? I also don't know how upset they would be if a day or two late but I would think any trading that you don't have the funds to back is about as crazy as you can get. You may get away with it for a while but one day it is going to burn you badly. I certainly wouldn't be able to sleep at night. Nothing wrong with using the leverage of margin if you know what your doing but t + 3 is just too risky imo.
