is it ethical to publish the same paper in a conference proceedings and a journal?
In computer science, the typical rule of thumb is that a conference paper may be 'upgraded' into a journal paper with at least 30% new material.  Certain subfields and other related fields covered by the IEEE may have different standards, however, and ultimately, the standard for this particular conference/special-issue pair is whatever the organizers have decided it is.  Since a 3 page paper is a very short form, though, the full manuscript will almost certainly be reasonable to submit to a journal.

It may not, however, be appropriate to submit to this special issue. Special issues associated with conferences are sometimes open submission (in which case, you can probably submit immediately), sometimes open to extended versions of any accepted papers (in which case, you need to wait to see if your paper is accepted), and sometimes only for a select set of invited papers (in which case, you need to wait to see if you are invited).  The organizers should have posted this information on their site: if they have not, you can email them to ask.