The T here refers to time, and the 3 refers to 3 days.
So T+3 = the 3-day time period that you have to wait after you buy or sell a stock. Yep, it takes three days from when your order is executed until you actually have the stock or the money in your hot little hands.
The T here is time, as in days. And the 3 (in T+3, for example) is, um… the number of said days. That is, if you put in an order on Thursday with your broker on a 3 day holiday weekend; call it Labor Day weekend, and your particular purchase is a T plus 3 settlement purchase, it means that you will have transacted on that thursday, but then you need to weight 3 business days until that transaction is considered, settled.
So in this case yeah we threw curve balls at your head. There was not only a weekend to deal with, but a holiday too. A good one. Love the hot dogs. So had this been a T plus 1 settlement, the trade would have settled….Yes, friday.
If T plus 2, then it would have settled…no, not monday, a holiday, so it doesn't count; it would have settled Tuesday. But its T plus 3, so it settles Wednesday.
Why all this lag in the era of light-speed computers? In the olden days, people waited for physical delivery of a dead tree with ink on it to consider a trade settled…
Used to be 5 days, stocks 3 days, and bonds and money market funds 1 to 3 days. If, however, you hear T minus 3…uh, it may be time to climb into your bomb shelter…