Did M3 employ a person that was employed by M1 and was employed by M2
SELECT count(*) WHERE {
?x0 a ns:people.person .
?x0 ns:people.person.employment_history/ns:business.employment_tenure.company M1 .
?x0 ns:people.person.employment_history/ns:business.employment_tenure.company M2 .
M3 ns:business.employer.employees/ns:business.employment_tenure.person ?x0
}