To solve for three variables you need three independent equations that use them.

Let x = the number of small pizzas
Let y = the number of medium pizzas
Let z = the number of large pizzas

Eq (1) x + y + x = 20
Eq (2) 5x + 9y + 12z = 181
Eq (3) 3x + 4y + 5z = 82

That'll get you halfway there. Good luck.

By the way, Blessed is right. Story problems are applied math. You need to learn to do this yourself.

Do your own homework. You will eventually be out in the workforce and you will be expected to be able to figure things out without running to coworkers or your boss for every little thing. Coworkers take a dim view of someone who can't do their own job and while I wouldn't do such a thing myself, I've seen people take some pretty clever revenge on such a person.