A is easily expressed in a CHECK CONSTRAINT on your timespan table, since each row can check itself for consistency without checking any other rows.
B and C are more complex and would require a reading information out of other tables. In this case (at least in SQL Server), one could use a user-defined function (passing it the values from the current row) to check against the other tables or you could use a TRIGGER which would check the other tables.
NB: When writing such a trigger, remember that a trigger fires once for a SQL statement. If that statement INSERTs or UPDATEs multiple rows, the INSERTED table will contain multiple rows, which should all be checked to match your domain constraints using an appropriate technique, like a JOIN.
BTW: Joe Celko had a good article recently on constraints.