Hi @Hiti Verma how are you?
Let me see if I can help you.
If I understand well your need the best approach is to have 3 different backlogs. Think in a way of each BU is a product so it needs to have 3 backlogs, one for each.
Even if the BU works togheter some time, you can link issues among the projects itself.
And for granular informations the three will help too.
I hope this help and reach your need.