How can I control the order of a query regarding to inner join
The easiest way to influence a query plan and make it run faster is to have good indexes and ensure the statistic on those indexes are up-to-date.  In your case have indexes on the primary and foreign keys in each table.  It would be best if you included b_come and c_type in their respecitive indeces, too.  (Sorry, I'm not strong on Informix; if the syntax allows the include clause or filtered indexes investigate and implement these.)

Wrapping the date() function around a_dt generally precludes any use of an index on that column.  If makes it "non-SARGable" in the jargon.  It may be better to structure it as

WHERE a.a_dt &gt;= '01/01/2012 00:00:00.000'
and   a.a_dt &lt;  '01/02/2012 00:00:00.000' -- or whatever the day after 1/1 is in your locale.