Is it possible to provide a list of allowed templates depending on a given key value ?

I would create the initial template list provider containing only common functionality. The point of the technique is to create the common structure, and modify it for each specific case.

Each of the three projects would then make the common functionality specific by passing arguments specific for that project.

So, do not create a single template list provider for all your templates, create one common template list and make it specific through parameters.

Regards,

Jostein