You need to link your table to table tblcomputer
SELECT dbo.tblComputers.Computername, dbo.tblComputersystem.Manufacturer, dbo.tblComputersystem.Model, dbo.tblADComputers.Description
FROM dbo.tblComputers INNER JOIN
dbo.tblComputersystem ON dbo.tblComputers.Computername = dbo.tblComputersystem.Computername INNER JOIN
dbo.tblADComputers ON dbo.tblComputers.Computername = dbo.tblADComputers.Computername