States parties is a term of art in public international law. It refers to the states that have signed a particular treaty. For example: states parties to the Treaty of Versailles.
It may not make sense from a purely grammatical perspective, but it is regularly used in international law. The first and second options are not commonly used in international legal material and would probably considered wrong if you are writing for a diplomatic or legal audience.
As an example: UNESCO, States Parties Ratification Status
States Parties are countries which have adhered to the World Heritage
Convention.They thereby agree to identify and nominate properties on
their national territory to be considered for inscription on the World
For reference, explained by Karel:
In international law it is a term of art as stated above. You find examples of this in international treaties such as Article 4(1) of the International Covenant on Civil and Political Rights (ICCPR) which reads:
"In time of public emergency which threatens the life of the nation and the existence of which is officially proclaimed, the States Parties to the present Covenant may take measures derogating from their obligations under the present Covenant to the extent strictly required by the exigencies of the situation, provided that such measures are not inconsistent with their other obligations under international law and do not involve discrimination solely on the ground of race, colour, sex, language, religion or social origin."