If your agreement with the Agents is for 3 years ( common Law agreement) then you can ask for the possession back As Common Law Agreements are contractual in nature, parties are bound as agreed in the contract. You need to go through if the term is for 3 years or till the sub-tenant leave the rented property?
So, you need to go through your agreement with the Estate agents to know the term of the agreement and when you can get the possession back.
If the term is of 3 years , you can take over the tenancy as their new landlord( provided there is nothing to provide otherwise in your common law tenancy agreement with the agents)
You can take over the tenancy with the sub-tenants by serving section 3 of the Landlord and tenant Act 1985, informing the he /she is in fact the new landlord, having taken over from the old one. This notice must contain the details of the new landlord, Name and actual address of the new Landlord.
you would also serve a notice under section 48 of the Landlord and Tenant Act 1987, informing tenants regarding the address for service of documents.
Serving these notices, does not result in a new tenancy .The original tenancy agreement that was created between the tenant and the previous landlord remains in place.