3 point select to line

.

This one creates a copy of your splineshape and delete all knots that are preceed and those that go after the selected ones. Idea is clear and simple. You need to modify it to work with multi-spline shapes.
upd I just realized that it works incorrectly if spline is closed and the middle vert is first one ;)
I guess it is not the only bug

(
	ksel = getKnotSelection $ 1 as array
 
	if ksel.count == 3 do
	(	
		shp = copy $
 
		for i = numKnots shp 1 to 1 by -1 where i > ksel[3] or i < ksel[1] do deleteKnot shp 1 i
 
		updateShape shp
 
	)
)

Poly verts indexes are not ordered so you can't tell which selected vert is the middle one. And because of that you'll get three valid variants of spline: 1-2 + 2-3, 2-3 + 3-1, 3-1 + 1-2