There are a lot of assumptions in your example. The big one is that there is a valid employer/employee relationship between the leasing organization and its employees. Another one is that 100% of the employees of A, B and C are now the "employees" of the leasing organization.
Given your presumptions, what you say is correct. The former employees of A, B and C are treated as employees of the leasing company, for the purpose of determining whether the plan of the leasing company passes coverage and nondiscrimination. This would be true even if, for some reason, the benefits of some of those employees were taken into account in determining whether a plan of A, B or C met the nondiscrimination requirements.