Placing pencil, highlighting a constraint, draggging a ”trailing” ghosted reference line from and placing your point where lifting pencil.
Rhino let’s you leave these points ”behind” with small snap-markers and it’s ghosted cordinate lines before returning to subsequently choose which markers highlighted to attach entity points to. Markers are lit up and placed if you stay momentarily at a chosen point.
Anyways, just snapping an entity point would suffice, instead of attaching constraints afterwards.
They can be seen as tiny coordinate-line snap-markers, almost like surface vector points to attach your ”3-point circle” or what-have-you to, which disappear when an entity has been assigned it’s points. They are called objectsnap markers and you can see their originate points through ghosted lines as well as light up an abundance of them as one whishes.