Hi Dave,
The Three Step, like most of the other fundamental patterns in basic Foxtrot, is at its core a component made up of basic groupings of 3 steps counted SQQ. However, the ISTD does not shy away from overcomplicating matters wherever possible, and they have certainly done so in the case of the Three Step.
When reading Alex Moore's or the ISTD's books, you'll see the 3 step as RLR, but what you're really seeing is steps 2 and 3 (the LF and RF), with that additional "first step of following figure". I suppose they're relying on the fact that any previous figure (typically ending in a Feather Step or Feather Finish) has that extra step, except that in all cases except for the Three Step, that extra step is supposed to represent the first step of the next figure -- except in the case of the Three Step. It's an odd choice indeed, but they must have their reasons.
The one thing you can count on is that every choice in the ISTD book is done with careful consideration and purpose, and I have no doubt that the ISTD had their reasons for presenting it starting on the second step. I myself never learned what that reason was, because whatever point they were trying to make with that choice is likely not important enough to override the clarity of a straightforward presentation. But I do welcome others to chime in, especially those who might have some insight into the history and inner workings of the ISTD syllabus.
In the end, just remember that this all comes down to a choice in the way the figures are presented. They are the same figures with the same exact construction, the only difference from one syllabus to the next being the exact moment they consider the starting and ending points.
In creating our syllabus on BallroomDancers.com, we too have been extremely deliberate with our choices. But some of ours favor simplicity and clarity over other considerations, since our audience includes a healthy serving of the general public. This is why, for example, you see fewer abbreviations in our charts (e.g. "Heel-Toe" written out, rather than "HT").
With the Three Step, then, it was a clear choice for us to illustrate the pattern starting with the leader's left foot, as it is for all intents and purposes the Foxtrot version of the Waltz LF Forward Change -- a 3-step pattern that connects the Reverse Turn to the Natural.
It is for this same reason that we consider the basic version of the Three Step to be danced toward diagonal wall, with the line-of-dance version being the variation. (The ISTD takes the opposite approach, I believe more for historical considerations). For us though, thinking of the basic combination of Reverse Turn, Change, Natural Turn, Change, as fundamental to most ballroom dances sets up a foundation for students that makes it possible to speed up learning through association.
We did, however, follow the ISTD's convention of including an additional step following any figure that ends with forward momentum. I've been told this was Alex Moore's way of illustrating that there is momentum that shouldn't be broken at the end of these types of figure, as well as committing the next figure to beginning in that direction. I don't personally think it's necessary, and to some it could even be confusing. But what we ended up doing is including it to avoid breaking too much from established convention, and clarified in every instance that the extra step is taken "as the first step of the following figure". That way you as a reader have a clearer idea of whether the last step of any figure is supposed to connect to or overlap with the first step of the next one.
I hope that helps clear it up!
Regards,
Jonathan
