gamma Virginis post-periastron reopening

said:
For the latest Hanwell report on Gamma Virginis - see:
http://www.hanwellobservatory. org.uk/ and follow the links.

Gamma Vir is not currently viewable from my location, but I intend to
take some images when it comes round again. I took some images early in
"totality", i.e. when the separation was below the resolving limit of
my 4 inch refractor.

In due course, I intend to establish the centre of mass and the
relative masses of the two from my own observations, just for my own
satisfaction (I don't claim to be equipped for precision astrometry).

However, it seems likely that Gamma Vir has a reaaonably massive unseen
companion (or the 19th Century measurements were erroneous which seems
unlikely). This would, of course, screw up an attempt to establish the
position of the C of M. If so, I wonder if there is a reasonably simple
rule (e.g. a geometric one) for extracting the position of a third
member of the group or of establishing the position of the C of M of
the group from observations of two members only?