You can join three tables by providing the join condition for each join. e.g.:

SELECT *

FROM sysobjects AS o

JOIN syscolumns AS c ON o.id = c.id

JOIN syscomments AS sc ON c.cdefault = sc.id

Probably you would like to read more on this at http://msdn.microsoft.com/en-us/library/aa213233(SQL.80).aspx

Regards,

Andras