Thanks for picking this up OpenBrain. I'd long forgot that I opened a bug about this but it annoys the hell out of me every time I try to use it on three dots. Especially since it changes from when you first try and then try again with the tool now active and get different
results. I get the feeling the white mice put this together and are up in there studying how primitive terrian humanoids deal with inconsistent data.
I often just give up after about 5 annoying attempts and create a different geometry with a line or something. ( I have quite a nice hires monitor and would not want to see it get damaged
Folks may like to refer to the original thread which I opened before the bug.
It seems to me a good point to start is the hover hint ( although this could obviously be changes too , along with the doc ).
"Create a symmetry constraint between two points with respect to a line or a third point (S)"
That is quite clear and covers both line and point symmetries. It probably will translate well into most language that are not based on reverse Polish.
Being consistent between line and point symmetry is desirable ( we don't want to have a three line hint with an if then else clause ) . Being consistent w.r.t. the state of the tool is essential. It seems to me that third element fills that need and just tidies up the current confusion with a minimum of change.
My vote is for 3rd element to be the centre of the symmetry.