Not very clear to me what you are trying to do.
If you have dates in several tables that you have joined, and want to return the date from the first table if that has one, and return the date from the second table if there is none in the first, and return the date from the third table if both 1 and 2 don't have the date, then it may be this:
COALESCE(t1.DateCol, t2.DateCol, t3.DateCol)
Post the query that you have now with some sample data and information about the tables, and surely someone on the forum would be able to help.