If the Delta variant is the “highly contagious” one...how come only 3 tested positive for it???
They likely all have the Delta strain. Only 3 have had the much more expensive gene sequencing test in addition to the standard PCR testing to phenotype the Delta from the Alpha or Beta strains. Since this is a superspreader event by definition it is nearly a certainty that the source vector was one person with the Delta strain who then spread it to one out of three of those in attendance.
The other take away should be that out of a 1/3 unvaccinated transmission rate the fully vaccinated breakthrough rate is 6/150 =4% so far and that is exactly what the much larger Israeli data set found as the breakthrough rate. This is encouraging news as none of the breakthrough cases are severe nor hospitalized with this Delta strain.