Example : The association table has "ForeignID" and "ForeignSource". Depending on the ForeignSource it will successfully retrieve data when join with multiple table.

Sample query I'm not sure would work or not :
Declare @UserInputForeignSource varchar(50)
Set @UserInputForeignSource = 'Customer'

SELECT * FROM AssociateTable t1
LEFT JOIN Customer t2 ON t1.ForeignId = t2.id AND @UserInputForeignSource = t1.ForeignSource
LEFT JOIN Vendor t3 ON t1.ForeignId = t3.id AND @UserInputForeignSource = t1.ForeignSource

Come to the next drinkies then ...
I'll get around to organising it at some point.