The fruits, if you collect all three, will give you a cart for free. If you don't collect all three, but manage to collect one or two, I believe you will get some coins for your trouble.
If you collect 100 coins within the Junimo cart game, I also believe that will grant you another cart.
Forgive me if this was already said, or if this is incorrect; it has been a long time since I played Junimo Cart...
Now... Is this mini-game hard?
Yes. I've completed Prairie King. And I'll tell you. That was a breeze compared to Junimo Cart. Junimo Cart is evil. If there is a devil in the Stardew Valley world, it's Junimo Cart. Evil. Evil mini-game.