Overnight financing for positions is when you keep positions open overnight, also referred to as a swap. If you keep positions open past rollover time (22:00 GMT) you will either earn or get charged the swap value. The value of the swap depends on the size of your open position, the instrument you are trading, your account type and the direction of your position. There are different swaps for long positions and short positions on each instrument.
Swaps apply triple on Wednesday, because that’s how the banks supplying liquidity to the financial markets apply them. This has to do with the way spot Forex contracts are settled. Spot Forex transactions have a 2-day settlement period. A spot FX transaction made on a Monday, settles on the following Wednesday. Trades from Tuesday settle on Thursday etc. But trades rolled into Thursday settle on the following Monday as there are no settlements over the weekend when banks are closed. Hence the reason for triple swaps applying to running positions at Wednesday rollover time.