Join three tables that share a common column
There are (at least) two ways to write FULL joins between more than 2 tables. Using FULL JOIN multiple times, the expression in the ON condition gets a bit longer but it's pretty simple:

SELECT
    COALESCE(t1.Hostname, t2.Hostname, t3.HostName) AS Hostname,
    t1.OS, 
    t1.Confidence, 
    t2.Manufacturer,  
    -- the rest, non common columns
FROM Table1 AS t1
  FULL OUTER JOIN Table2 AS t2
    ON t2.Hostname = t1.Hostname
  FULL OUTER JOIN Table3 AS t3
    ON t3.Hostname = COALESCE(t1.Hostname, t2.Hostname) ;


Using first a UNION of the common column, then LEFT joins:

SELECT
     u.Hostname,
    t1.OS, 
    t1.Confidence, 
    t2.Manufacturer,  
    -- the rest, non common columns
FROM 
    ( SELECT Hostname FROM Table1 UNION
      SELECT Hostname FROM Table2 UNION
      SELECT Hostname FROM Table3
    ) AS u 
  LEFT OUTER JOIN Table1 AS t1
    ON t1.Hostname = u.Hostname
  LEFT OUTER JOIN Table2 AS t2
    ON t2.Hostname = u.Hostname
  LEFT OUTER JOIN Table3 AS t3
    ON t3.Hostname = u.Hostname ;