A Swap is a fee for keeping a position overnight. It represents the difference in credit interest rates for currencies used in trades, therefore Swap rates may vary from time to time depending on the rate.

The triple Swap, or 3-day Swap occurs on Wednesdays, because most instruments need two business days (T+2) to be settled (for all the financial transactions to be completed).

If you roll the Wednesday position over to Thursday, the Swap rate will also account for rolling the position over the weekend – thus the triple rate.

You can check out how Swaps are calculated, per instrument, here.