An antibaryon composed of two antiup quarks

and one antidown quark would have a charge of

(1) + 1e (3) 0e

(2) - 1e (4) - 3e

So we want to know what charge would an antibaryon composed out of one antidown quark and two antiup quarks have. Antidown quark has a charge of + 1/3. Antiup quark has a charge of - 2/3. So when we add all of them: 2 * (-2/3) + 1/3 = - 4/3 + 1/3 = 3/3 = 1. So one antibaryon has a charge of + 1.