The ISTD technique book is confusing for a non-expert, and sometimes for an expert. You are correct that the 3 step starts on the left foot. I believe (my syllabus book is upstairs), however, that often steps in foxtrot are shown ending with an 'extra' step. I think this is in part so that they can discuss inline/outside positions, etc. In short, you need to know in order to know. Looking at the Precedes and Follows for the steps should help. Also check out http://ballroomindex.weebly.com/ they have an interesting presentation for this: http://ballroomindex.weebly.com/choreography.html

If you want another excellent example of the technique book having...er... issues... Look up the Open Finish in Tango.