Swap a.k.a interest is paid or charged (depending on which pair you trading) every day...365 days a year.
Triple swap/interest rates are paid or charged in the roll-over period on Wednesday night to account for the settlement of trades over the weekend where no swap rates are paid/charged due to the market being closed.
Why and when market chose Wednesday to be the trible swap rate? I do not have the answer to that because I have not yet conducted a search on that subject....in other words, too lazy to do that!
You can avoid swap rates by asking and qualifying for an Islamic Trading account from your broker. However, some brokers charge a "maintenance fee" for a swap free trading account.
Because they are making up for the weekend where they can't charge swaps, they charge triple on a Wednesday instead. Even if you open a trade on a Wednesday and hold into Thursday, you will be charged triple.