Altium error: Net contains multiple output sheet entries
For now you have two buses with same name, that is how Altium sees it, and it makes sense. ADC[0..2] on both sheets.
You can not make Output for bus containing 3 signals, from two different sheets. Your A2 sheet contains only one signal and A3 two signals, so solution is to make Bus entries at top level and connect those signals with outputs from sheets A2 and A3. You can name your signals as ADC 0, ADC 1, ADC 2. ADC 0 is output from A2 and is connected to the bus over bus entry, same is with ADC 1 and ADC 2. And input to A1 can be same as it is now.