The third brother dies before acquiring the full practical obligations of a mature person. His circumstances cannot simply be measured by the obedience or wrongdoing of the other two brothers.
Taklīf is religious obligation subject to its relevant conditions. The initial question concerns whether accountability attaches to the child; the further questions of Paradise, reward, divine grace, and matters beyond observation require their own evidence.
That is what the thought experiment exposes. If the third brother did not live to perform mature obedience or commit mature wrongdoing, what basis exists for assigning him the precise reward or punishment of the others? The larger dispute concerns the supposed obligation of aṣlaḥ upon God.