@ Burrman:

<< If you use the arc by 3 pnts command, select the 2 pnts first, then in the 3d perspective viewport there is an "OnSrf" snap to get the 3rd selection on the surface.>>

Thanks for the reply, but you would have to know where to precisely place the third point (since you could place it anywhere on the surface). And I don't have that information.