Binance, one of the largest crypto exchange platforms with its CEO and founder being Changpeng Zhao, has decided to remove three biggest stablecoins USDP (Pax Dollar), USDC (Circle USD) and TUSD (True USD).

What about the owners of those crypto who own those stablecoins on the Binance platform?

Binance are to convert any of the balances/deposits of these coins to their own, the BUSD (Binance USD). (From 29/9/2022) 

Why are they doing this?

According to @EvgenyGaevoy's tweets he believes that liquidity doesn’t have to be split between multiple stablecoins, making market makers’ job easier.

You should still be able to deposit and withdraw USDC seamlessly to/from binance. Before this change you would need to convert it to BUSD/USDT and then use it to trade margined perps Now you can skip the conversion step both ways. The one stablecoin that is clearly not part of this is USDT.