..

You have a couple options:

1. If your account mapping depends on the U3 dimension, at the data source level you can concatenate the account with the U3 field together in the account field. Then in the account field you would have everything you need to build out the mapping OR:
2. You can do a composite mapping which means you can base the Account mapping off another field like the U3 mapping

Which one you chose depends on how you want to maintain your mappings and performance. 

Here is an example of a composite account mapping for the U2 dimension where the U2 dimension mapping is reliant about the A# account dimension:

Again, either way should work so it is for you to decide the best approach for your use case.