Hi @ReddyIshanvi
Use case mentioned above is achievable using User, groups and permissions. What you can do is create 3 separate groups and assign the individual users accordingly.
UserGroupBrand1- It would have only permission on /content/site1 and /content/dam/brand1 Dam assets.
UserGroupBrand2- It would have only permission on /content/site2 and /content/dam/brand2 Dam assets.
UserGroupBrand3- It would have only permission on /content/site3 and /content/dam/brand3 Dam assets.
Based on user access, they would only see the content relevant to their site and DAM.
Hope this helps.