Use this
SELECT dbo.tblComputers.Computername, dbo.tblComputersystem.Domainrole, dbo.tblComputers.Domain, dbo.tblComputers.LastknownIP,
dbo.tblComputersystem.Manufacturer, dbo.tblOperatingsystem.Caption
FROM dbo.tblComputers LEFT OUTER JOIN
dbo.tblOperatingsystem ON dbo.tblComputers.Computername = dbo.tblOperatingsystem.Computername LEFT OUTER JOIN
dbo.tblComputersystem ON dbo.tblComputers.Computername = dbo.tblComputersystem.Computername