Can anyone explain in an accessible way why split A into A1, A2, A3, if, as a result, all these 3 keys will belong to one person?
In this way, the person A owns 3 out of 6 keys and it won't be possible to make any transaction from the wallet without his permission. Even if all persons B, C and D agree to make a transaction, they will still need person A's permission, since the wallet is a 4 of 6 multi-signature wallet. This is what OP is trying to achieve.