I took the trouble to hunt for the video where Marcus Hilton does a Curving Three Step.
Cdroge. wins the argument. There are two straight steps by the man taking the floor boards as a guide. What can be deceiving is the turn of the body to the left. But the second step goes straight ahead in fact it looks like a step to the side. but is straight ahead taking the first step as a marker. Does that make sense.

I don't see how the first two steps can be straight. How would you lead the lady into the curved 3-step. You would have strong CBM, but there is a need to 'cut the lady off' so she realises it's a curved 3-step. If all the turn is made on the last step, it would be more like a 'dog-legged' 3-step!!
I think a description from Geoffrey Hearn's book had 1/8 of turn on second step. That sounds about correct.
Remember also the figure can curve to the right. Been dancing curved 3-steps (to the right) for years. Not perfectly of course, but getting better

"There are two straight steps by the man taking the floor boards as a guide"

But what was the direction of the preceding step 3???

To have two straight steps would imply that the footprints of three steps (3, 1, 2) can be connected by a straight line. If that isn't the case, then you haven't taken two straight steps.

Anonymous. He is facing straight down the Lod parallel with the floor boards. This how I can tell which direction he is stepping by taking the floor boards as a guide.

"Anonymous. He is facing straight down the Lod parallel with the floor boards."

You've answered the wrong question.

I didn't ask the alignment, I asked the direction of the previous step.

You must then further consider the position (location) of the step. Can three successive footprints be connected by a single straight line? Or do they make two line segments with an angle between them.

Anonymous. I am not sure what the problem is or what you are driving at.Put it this way. I can step straight ahead with my Left and straight ahead with my R. If turned my Right foot 1/4 of a turn to the left on arrival it was still a straight step. I doubt whether it would be necassary to do that. It's just to illustrate what a straight step can be.