triple column design, in a flexible way

Hi!
Now I have a triple column design, with the right sidebar(sidebar.php) in a separate php-document and the left sidebar coded into the code of the header!?!
Both are available via the admin widget menu.

Because the left sidebar is coded into the header it is of course not in an separate document in the admin/appearance/editor(is it appearance? I translate from swedish), but from the header.php.

I would like to lift out the left sidebar and put that one as well in a separate php-document. Of course I still want both to be accessible within thw widgets menu, and now BOTH visible in the
admin/appearance/editor as, for example leftsidebar.php and right sidebar.php

Is this possible, or is WP not supporting that directly?

Hope 4 answer.
/Johan