In foreign exchange trading, overnight interest refers to the interest cost or income incurred by holding a foreign exchange position overnight (beyond the trading day). The foreign exchange market is a 24-hour continuous trading market, so at the end of the trading day, traders who still hold open foreign exchange positions will incur overnight interest. The calculation of the overnight interest is based on the difference in interest rates of the currency pairs. Each currency has its corresponding interest rate, which is set by each country's central bank or monetary policy-making body. When there is a difference in interest rates between two currency pairs, holding a position in those currency pairs overnight results in an interest expense or income.
With regard to Wednesday (foreign exchange, gold, silver, stocks) and Friday (commodity and index) charging 3 times overnight interest, this is usually because the delivery day in the foreign exchange market is T+2 (the second working day after the trading day) day), that is, if a trade position is held overnight on Wednesday, delivery will take place on Friday. To reflect this extended holding period, 3x overnight interest is charged on Wednesdays. Different exchanges may have different regulations on the overnight interest charging rules for foreign exchange and indices. Some exchanges may require 3x swaps on Wednesdays, while others may require 3x swaps on Fridays.
To put it simply, the 3 times overnight interest is to make up for Saturday and Sunday (the bank is closed and cannot be settled).