Answer to your query can be found in many discussions we had earlier. Still, I am providing it for your ready reference.
As Regards to PF:
On reaching 20 or more employees (headcount irrespective of salary limit).
As Regards to ESI:
In all states & U/T other than Maharashtra, HP & Assam - On reaching 10 or more employees (headcount irrespective of salary limit).
In Maharashtra, HP & Assam - In factories, 10 or more coverable employees & in establishments other than factories, 20 or more coverable employees.
As Regards to Gratuity:
On reaching 10 or more employees.
For PF & ESI, the definition of an employee includes contractor's employees as well. For Gratuity, contractor's employees are included, and no registration of the establishment under it is required.
Value addition to the above will be appreciated.
Regards.