Here is the official tier list for all 21 selectable characters for Mario Hoops 3-on-3 made by myself and badBlackShark. As fun as the game itself is, it's not as balanced as it potentially could have been. While being a Basketball game, many characters are left at a disadvantage towards others due to their lack of ball movement, special shot, ground-pound recovery and dunk speed animation. While other relatable points could be added, it should be understood that this tier list is based mainly on these 4 foundational technicalities.
Please bare in mind that this is the official tier list for the game in general. When it comes to speedrunning, you may want to customize your team your own way just as you would do the same if you were playing it casually. This list should not discourage those from playing a specific character that they like if they happen to be low tier. It's purpose is to simply show the difference between each character. In the end, it's really the player that makes the character great; however, some characters will come easier to others without a doubt. So here it is:
1.) Black Mage 2.) Paratroopa 3.) Ninja 4.) Mario 5.) White Mage 6.) Cactuar 7.) Luigi 8.) Bowser Jr. 9 & 10.) Diddie Kong / Dixie Kong 11.) Birdo 12.) Yoshi 13.) Waluigi 14.) Daisy 15.) Peach 16.) Bowser 17.) Donkey Kong 18.) Wario 19.) Boo 20.) Moogle 21.) Fly Guy
Sincerely, Hoopla Fish