Coins are dollars ($$$) in Super Mario Bros. 3
Coins have been the currency of the Mushroom Kingdom since the original Super Mario Bros. for the NES in 1985. Within games, the number of coins collected by Mario has always been denoted by a "coin" icon in the in-game user interfaces.
However, after recently playing Super Mario Bros. 3 on the Nintendo Switch Online - which features NES retro classics, I noticed that the coin is actually represented by a dollar sign $ icon. This is a little unusual, since it implied that coins are presumably "dollars" in this game. At least in mainline Mario games the currency was always just called "coins", unlike in Zelda where they are rupees.
Super Mario Bros. 3 on the NES showing the "$" for coins
In every other Super Mario game, coins are denoted with a coin icon, even in the original SMB. I'm not sure why they decided to change it for SMB3. Its successor, Super Mario World for the Super NES went back to using a regular coin icon, and even the SMB3 remake in Super Mario All-Stars got changed back to a coin icon.
In the Super Mario All-Stars remake, the coin icon is back
We'll never know why... Maybe, Nintendo was money hungry and was anticipating that the release of Super Mario Bros. 3 will be like printing money, and it sure was, since SMB3 was one of Nintendo's all time best sellers!