Two poeple leaving a joint tenancy agreement

Hi all. I have two tenants that have given me a month’s notice to leave my flat. However there is joint tenancy agreement in place and the third tenants wants to stay.

I would actually prefer to bring this tenancy agreement to an end and either start over, using some time to refurb or sell it.

Am I write in thinking that the joint tenancy agreement comes to an end for everyone if one person chooses to leave? What do I do if the third tenant refuses to leave?

Yes, the tenancy ends when one or more wants to leave. If the 3rd one stays you will have to evict them or start again with referencing and affordability checks.
Are they related ?

It only ends if the tenancy is periodic. A tenant notice served during the fixed term is of no effect.