You can start as small as $2-3 by trading small alts and flip them for substantial profits.
The only thing you should keep in mind while dealing/trading in cryptocurrencies is that you should bet only what you can afford to lose.
Good luck!
To me, I feel this $2-$3 is kind of small to make some kind of profit in trading. Yes, there is no required amount needed for trading, but there is also a limited amount that is minimum for trading. We all know that one should invest or trade what they can afford or use the money they are willing to use but using pennies to trade is a wasting of time. You will end up buying shitcoins you may not be able to sell.