I've been trying to create a custom component which takes three tangents as a vector and creates a circle from that. I didn't succeed :-(
but I know it must be possible since Rhino has this feature in the circle commands. I just don'T know how to access it via .Net
that feature isn't exposed. In fact, this is not possible, you cannot define a circle with 3 tangents. You can define a circle through 3 points (provided they are not coincident and not colinear), and you can define a circle through two points and a single tangent. I think the command in Rhino you are referring to is the option in _Circle which creates a circle tangent to three existing curves?
I do not think that function is exposed, and I can tell you it's incredibly difficult to get that work, especially if you have no mouse-picks to guide you towards the correct solution.