As per the description, I understand that you would like to opt for any of the below ways:
1. Merging data in AAM from 3 different business lines.
2. Merging data from 3 DMPs into 1 DMP.
I would like to update you that merging data 1 or more AAM accounts is not possible.
Within your AAM instance, you can have multiple datasources which will capture data from individual business units and within the same AAM instance, you can merge data from these three datasources as-n-when needed.
I hope this helps, please let me know for any additional question on the given matter.