Yes labor (PERM) has nothing to do with the employee. The same approved labor can be used for any of their prospective employee. Upto that stage you are OK.

When I-140 comes to play, employer has to present many supporting documents including company's financial history and revenue. It is not like PERM. It is totally emplyee specific with that particular employer.

Hence, I am pretty much sure you can not move forward with I-140 being in third company. However, I am not a lawyer though.