During human-cobot interaction, transitions often occur between free mode operation and constraint surface following. This paper considers the following typical scenario: an operator of a three-dimensional (e.g. x-y-z or x-y-θ) cobot commands a "join" from free mode to a one-dimensional constraint path. To ensure that this transition is smooth and responsive, a "blend path" is computed that joins the cobot's initial location to a location on the constraint path. The blend path must match tangency and curvature at each end, and must also exhibit continuous curvature. A novel "triple-clothoid" algorithm is developed that meets these requirements. In addition, we demonstrate that the triple-clothoid is particularly amenable to x-y-θ systems via implementation on a planar cobot.