SSRS report to coalesce separate data sources into one "complete" record

So, let's see if I have this straight.

You have three tables, which when combined make a complete record.

The keys in these tables are sometimes NULL or incorrect.

ex:

(table)        ID      Name
1       1               John
2       NULL    John
3       2               John

So then, how can you tell which ID is the correct one?

The real solution to your problem seems to be at the data entry end. Adding constraints or rules to the input might prevent the issue going forward. You will, of course, still have to clean up your bad historical data.