It really shouldn't be very hard, but of course it helps to have had some practice and gained some decent cards first. However, the championship players are all novices, and the third round is the easiest of them all. Even better, you can save between every round, so just save, play round one, save, play round two, and play round 3. I'm pretty sure they kept in mind that not everybody would have played the card game, so they made it fairly easy in the championship.

Choose the cards with the highest first digit, if you have any above 0, and try to get a healthy balance of cards with P and M as their second digit. Use the P cards against opponent cards where the 3rd digit is lower than the 4th, and M cards against opponent cards where the 4th digit is lower than the 3rd.