Looking to find a third person to join two others with a new tenancy

Hi Really hoping you can help.
A tenancy agreement that I have in place with an estate agency is coming to an end (thank god) and I am seeking to get new tenants for my three bedroom flat via Open Rent. One of the existing tenants wants to stay and he has found a friend who wants to move in. I want to create the new tenancy agreement in Open Rent but we are seeking another tenant to join the existing duo, hopefully via Open Rent. When posting the property on Open Rent how should I advertise it? Should I say room in a flat or the entire flat. I just don’t know. Any help greatly appreciated.