Nope, I think that's not how it works... My bank told me if I paid within the tradedate plus 3 business days I won't be charged interest at all. But if I don't have money after the 3rd day then they will charge me a hefty 21% interest.
What you just said is consistent with what I described. They are concerned with settlement dates. So yeah, you have 3 days.
I'll use an example to make it more clear:
If your purchase on Monday and sell on the same Monday, then both the purchase and sell will settle on Thursday in which case you will not be lacking sufficient funds and will likely avoid the interest fees.
However, if you were to purchase on Monday and sell on Tuesday, then what will happen is your purchase will settle on Thursday and your sell will settle on Friday. You will be charged interest from Thursday to Friday.
What you said in your original post implied that if you sell before the purchase settlement date (Thursday) that you would avoid interest and what I'm saying is that's not the case. It's not enough for you to sell before Thursday -- you actually have to have your sell settle on Thursday, which means you'd have to sell on the same day you made the purchase.
I'm not 100% sure this is how your bank will deal with you as I'm not familiar with how Canada (or your bank) handles these situations. What I described is what would be the logical approach, but of course that's not always the correct approach.