Long tenancy query

Tenancy Types and Management

I have tenants who want to stay up to three years - they require a fairly tight agreement which stipulates there will be no "exit" clause for the landlord (other than the usual defaults say, of unpaid rent, unsocial behaviour etc)
The property is an investment property and it is highly unlikely the landord will need to take the property back to live in, for instance.
Is there any other form of agreement other than continuing on a yearly AST?
What are the pitfalls for tenant and landlord alike?
thank you.