To win is to be #1 in a competition, or to receive an award.
▶️ My soccer team won the game 3-1.
▶️ I want to win the lottery!
▶️ John won a prize in the science competition.
You can win a game, a race, a match, a competition, or the lottery. You can also win
a medal (like in the Olympics), a prize, or an award.
To earn something is to get something in exchange for your work or effort, for
example: a salary.
▶️ Sarah is a famous lawyer; she earns a lot of money.
▶️ I’m not rich, but I earn a decent salary.
▶️ My bank account earns 2% interest per month.
To gain something is simply to get or increase – not necessarily because of action:
▶️ I’ve gained five pounds since I stopped exercising.
▶️ Jack’s car gained speed as he drove down the mountain.
▶️ The company is gaining international recognition.