Where a registered society or branch is entitled in equity to any hereditaments of copyhold or customary tenure, either absolutely or by way of mortgage or security, the lord of the manor of which the hereditaments are held shall, if the society or branch so requires, admit not more than three trustees of the society or branch as tenants in respect of such hereditaments, on payment of the usual fines, fees, and other dues payable on the admission of a single tenant.