SELECT query

I have three table called table1, table2, table3 all having fields like:
table1 contain id and username and id referencing table4
table2 contains id and parallel_username and id referencing table4
table3 contains id and other_username    and id referencing table4
table4 is the main table containing id as primary key
I want to develop a search engine which shows unique record after searching 
all the three table but shouldn't contain multiple answers for same SEARCH for
username
Pls help me,

-Regards,
Sharmad
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